Banedon did these for Selesnya and Mono-U Tempo; so I decided to chime in with one for my favourite deck of the season.
Card choices and their reasonings
Staples
These are the cards that I think should be in every single Golgari Midrange deck in a significant number. They're hard or impossible to replace and form the basic gameplan.
Llanowar Elves: 4 mainboard.
The 4-of Elves are actually not as unequivocally thought of as correct as it seems at first glance. Playing them gives you a big advantage in getting your finishers like Vivien Reid and Carnage Tyrant (or even Find // Finality) online a turn (or sometimes two) ahead of the curve. It also greatly helps in casting Jadelight Ranger. However, there are some match-ups in which the card impacts the board too little, and it's also a pretty bad topdeck lategame in a deck that ideally wants to improve their topdeck scenarios. I personally think that they are just too important not to play, but am open to sideboarding some or all of them out in matchups that will go super-late. I should also mention that there are decks that run only 3 or even none at all.
Merfolk Branchwalker: 4 mainboard
This is the single best 2-drop for the deck. Grants card selection/advantage while always providing a relevant body is really good.
Wildgrowth Walker: 2-4 somewhere between mainboard and sideboard
Highly matchup-dependant and somewhat controversial. It really helps against virtually any aggressive deck as well as Phoenix; but generates too little impact against many other decks and is often the first card to go to the sidelines. It also gets awkward if you draw it and none of the cards to go with it.
I've seen people run 4 mainboard and I've seen people run none main and 2 sideboard. Most decks I've seen run 3 mainboard, which I agree with; you draw it consistently enough without having to deal with awkwardness too often.
Jadelight Ranger: 4 mainboard
Absolute staple for similar reasons to Merfolk Branchwalker. Helps you hit land drops, gets creatures in the yard for Find, digs you towards what you need.
Ravenous Chupacabra: 2-3 mainboard.
Great in any even remotely creature-based matchup, can be awkward in multiples though. More of a removal spell that generates a 2/2 body and can be recurred back with Find or Memorial to Folly. Personally going back and forth whether 2 or 3 is the right number, currently leaning towards 2.
Vraska's Contempt: 2-3 mainboard, up to 4 in the 75
This removal spell's value has been on the up and up ever since the start of the meta. What started out as 1 or even 0 copies has gone to a point where most lists mainboard 3, sometimes with number 4 in the board. With Arclight Phoenix decks somewhat out of the picture the fourth copy is overkill IMO, but not running at the very least 2 main 1 side is crazy IMO.
Vivien Reid: 2-3 mainboard
Vivien is the engine to keep the Golgari train running. Her -3 deals with everything Carnage Tyrant cannot (aside from The Immortal Sun), her +1 finds the aforementioned dino or whatever it is that you need, and her ultimate is insanely threatening. Never run less than 2 maindeck, personally I've loved copy number 3 as well.
Carnage Tyrant: 2-3 mainboard, up to 4 in the 75
Doing its name proud. He's big, he's bad and he'll demolish the opponent in short order unless they jump through hoops. Also, guess who's back after Find or Memorial to Folly. IMO you should never run less than 3 in your 75, and it's not far-fetched to just main these 3; I may sideboard out one if I feel like I need other cards more.
Find // Finality: 3 mainboard
Nice, flexible card that acts as a sweeper or as card advantage depending on what you need. Also makes one of your creatures into a legit threat (a 3/5 is nothing to scoff at, as is a 9/8 Carnage Tyrant).
3 is the number needed to draw it consistently especially game 1, though I sometimes board down to 2 or even 1 if I feel like neither half does enough in the matchup.
Maindeck filler or sideboard cards
After adding all your staples, these cards are all decent options to round out your maindeck. Often, these cards also make decent sideboard cards as you may have a 'one or the other' decision turn into a 'Why not both?'.
Druid of the Cowl: 0-2 mainboard
This card is often used to fill out the 2-drop slot; especially in decks that don't run Wildgrowth Walker. It can work in conjunction with Llanowar Elves to get your big plays out even earlier and survives Finality.
Seeker's Squire: 0-2 mainboard
Another alternative to fill out the 2-drop slot. Squire is a lot worse than Merfolk Branchwalker and should only be used in conjunction with 3 or more maindecked Wildgrowth Walkers to make sure they trigger often enough. 2/3 and 1/2 are just much, much worse statlines for 2 mana than 3/2 and 2/1 in Constructed.
Cast Down: 2-4 in the 75
Your most efficient removal spell against aggro of any kind, and your best answer to hate cards like Tocatli Honor Guard. For this reason alone, you need a decent number somewhere. However, since it's really bad when it's bad (against Control of any kind and even against Boros Angels game 1 since they only run Tocatli in the SB), I like to sideboard the bulk, if not all, of them Currently running 3 sideboard, could see mainboarding one or two is the meta gets more aggressive.
Assassin's Trophy: 1-3 in the 75
Cheap catch-all removal spell that should not be played early; also gets worse in multiples. Makes it easy to double-spell later in the game though. I personally am a fan of the 1 copy mainboard, 1 copy sideboard split; some decks run 3 somewhere in the 75 but I can't think of a single matchup where I'd want 3 Trophies in my deck. On the other hand, I'm not a fan of running flat out none, because the card does provide a great deal of flexibility and is impactful even as a 1-of, giving you the ability to even destroy lands that could otherwise get problematic (Detection Tower, Arch of Orazca, the flipsides of Search for Azcanta or Legion's Landing or something as simple as Memorial to Folly).
Midnight Reaper: 1-2 mainboard; up to 3 in the 75
IMO your best secondary 3-drop, mainly for its ability to add to the board while giving you more than due compensation for an opposing sweeper (bar Settle the Wreckage, but you can play around that and get a few lands out of the deal) and makes combat a nightmare for the opponent. Running too many can lead to your ife toal depleting too quickly, though.
Thrashing Brontodon: 0-2 in the 75.
3-drop that is somewhat matchup-dependant. Having access to enchantment hate as well as a card that survives blocking most W Weenie threats as well as Deafening Clarion is almost mandatory, but the 3 slot is crowded. I personally like them more in the board.
Plaguecrafter: 0-2 in the 75.
Another possible secondary 3-drop. It's your cheapest answer against opposing Teferi, Hero of Dominaria and Niv-Mizzet, Parun (and cannot be answered by Negate or Spell Pierce), hits even hexproof threats and can be brought back in a pinch. It's highly match-up dependant though, which is why I like them in the sideboard, but some lists maindeck them.
District Guide / Elvish Rejuvenator: 0-1 mainboard
Budget options or options that function as a 7th 3-drop that are somewhat useful in any match-up. Generally a little too low impact, though.
It should be noted that a budget version of the deck can run District Guide and some small number of Golgari Guildgates to replace Jadelight Rangers and either Overgrown Tomb or Woodland Cemetery; you should not go overboard with this since Jadelight is a really important card, though.
Golgari Findbroker: 0-1 mainboard
This card has been on the decline. At the start of the format, most people would include 2, or at least 1 main 1 side; then most people quickly figured out that running more than 1 is too slow in many cases; currently most decks run none at all. There's just too many better options at your 4-drop slot, and while the interaction with The Eldest Reborn and Memorial to Folly is cute, it's too slow to be a staple interaction.
I will note that Findbroker + Eldest Reborn is a budget-friendly variant, though.
Vraska, Golgari Queen: 0-1 mainboard
The GRN addition, Vraska is quite a lot worse than Vivien. I still like to run 1 because her -3 makes her the best answer to dangerous W threats such as Benalish Marshal or History of Benalia, as well as other cheap yet dangerous cards, and her +2 has some utility as well; but she gets absolutely awkward in multiples and sometimes you don't even want the first one.
Karn, Scion of Urza: 0-2 in the 75
Karn will prepare your silver bullets while granting card advantage at the same time. Howevr, it's hard to make room for him without getting too passive. I currently am running 1 main; could see number 2 making the main if I sideboard (or cut) Vraska, Golgari Queen.
Doom Whisperer: 0-2 in the 75
The single most controversial card in the archetype.
5 mana 6/6 flying trample are ferocious stats, and the added bonus of being able to surveil in a deck that cares about the graveyard seem too good to pass up? Well, the big problem here is that it dies too easily. The ability to surveil for life does not make up for the fact that it can be targeted (and, in most instaces, killed) by virtually any removal spell as well as Vivien Reid. It's good against some matchups, but these matchups seem to be on the decline, which is why I personally went ahead and cut them entirely.
That said, some lists jam the full 4 mainboard and eschew Carnage Tyrant.
Vraska, Relic Seeker: 0-1 in the 75
I'm a pretty big fan of diversifying threats, and that's exactly what this PW does. Instead of running a fourth Carnage Tyrant (or a third Vivien), you run a 'silver bullet' type card that can also win the game, and in some instances works better than the aforementioned wincons. Again, I like her as a 1-of main or side, currently I've got her in the board, but shes coming in on a regular basis.
Sideboard cards
These are cards that should only come in out of the sideboard. Most of the time they don't actually contribute to your main deck plan; instead they work against an opposing deck's plan and if you are not facing that specific deck, they're bad; but that's why they are in the sideboard after all.
Duress: 3-4 in the sideboard
The bread-and-butter sideboard card for Golgari. In the matchups it's bad, it's REALLY bad, but it is a very useful tool in the match-ups where it's good, dismantling the opponent's plan. There's some controversy about bringing it in in specific match-ups; but there's little controversy about it deserving these sideboard slots. Never run less than 3, I personally run 4 but can see arguments why you'd only want 3.
Deathgorge Scavenger: 0-3 in the sideboard.
This is mainly good in match-ups where Wildgrowth Walker is good, exiling opposing Phoenixes and gaining life. If you face these match-ups a lot, consider bringing in some more Scavengers, but don't go overboard.
On that note, a shoutout to Blood Operative, which can be a 1- or 2-of in decks with 4 Doom Whisperers (likely taking this slot), but is not good enough with less.
Reclamation Sage: 0-1 in the sideboard.
Alternative enchantment hate if you dislike Thrashing Brontodon. Sage is 1 mana cheaper and leaves a 2/1 body behind, Bronto is better if the opponent has no enchantments out and can actually be recurred and sacced again during the same turn.
It is a bit too narrow to warrant more than 1 copy, but that 1 copy can definitely have a big impact.
Golden Demise or Ritual of Soot: 2 sideboard
I listed both of these sweepers because they compete for slots and you have to decide which one to take. Golden Demise is cheaper and doesn't necessarily hit your stuff, Ritual of Soot answers boards more cleanly (especially through buffs). I personally like Golden Demise a little better, but can see both sides here.
The Eldest Reborn: 0-2 sideboard
Really slow, but highly effective in the matchups it's good in. I've seen (and played) decks with up to 3 copies, but concluded that anything beyond the first copy is probably too clunky. That said, the first copy is highly effective in basically any non-aggro matchup.
It should be noted that you can mainboard one copy if you are on a budget.
Landbase
I'll write about the landbase in a secluded area. Since we're more or less talking deck concept instead of actual deck, I won't go as much into detail as Zerris or me used to back in the Duels subforum, but I will give guidelines in regards to landcount and nonbasic lands that can or should be run.
Landcount: 23-24 lands.
Generally speaking, GB Midrange is a very mana-hungry archetype and wants a high landcount. This is somewhat mitigated here by the presence of mana elves and Explore creatures (as well as the planeswalkers to an extent); though these still need a decent land count to function. I personally like the 23-24 range, depending on the lands you play, but have seen lists that run 25.
Nonbasic lands to be on your radar:
Overgrown Tomb: 4 mainboard
The single most important dual in the deck; makes sure your Woodland Cemetery comes in untapped and lets you cast Llanowar Elves turn 1 while also providing
.Woodland Cemetery: 4 mainboard
Produces Green and Black and comes into play untapped basically from turn 2.
Memorial to Folly: 1-3 mainboard
The ability to recur Carnage Tyrant, Chupacabra and other dangerous threats makes this land a mainstay; it also prevents flooding out. Entering tapped can lead to awkward situations, though. I personally think 2 is a good number as it otherwise competes for resources with Find too much, but you basically always want one on the board (It's also nice to leave up Contempt if you can leave up Folly at the same time).
Golgari Guildgate: 0-1 mainboard
If you need extra fixing, this is there. I persoanlly think it's too much worse than the other duals that I'd rather not start it, but there are people who think otherwise. Also, it's there for you if you're building on a budget.
Detection Tower: 0-1 in the 75
You generally don't want many lands that cannot produce coloured mana, but if you want to run one, this is it. It helps you get past opposing Carnage Tyrant and Dive Down, which you have little ways to beat otherwise.
If you want to put a land into your sideboard, this would be the one; you can also maindeck it if you're running at least 24 lands (running it with 23 leaves you with too few sources of coloured mana for my taste).
I should mention that there's no need to run Field of Ruin in this deck since you have enchantment removal to kill the flip enchantments and also Assassin's Trophy in a dire emergency.
Well, that's it for the primer on card choices.
Not sure whether I'll also write up a sideboard guide (I generally like to do that on a specific example, as the SB plans differ quite a bit especially on Golgari MR).
I also wrote a sideboard guide.
If you feel like I missed cards or should explain some points a bit more, hit me up.
EDIT: Fixed some errors, adjusted a couple numbers (thanks tmp_tmp)
EDIT2: Sideboard guide
. The final thing that convinced me was vivien can’t find her and when I run more planies and removal, I sometimes get nothing with vivien or only lands.