What are the current decks to beat?

I tend to play Casual and Tavern Brawls mostly, so I really don't see much of the top tier decks as I get to Rank 20 and then stop. Thought it would be cool to see some of the 'decks to beat'.

~SE++

I tend to play up to rank 20 too, here's some of the major archetypes i encounter:

All Taunts
This deck likes to protect itself by running every taunt card. Usually its not a big trouble if you're running any efficient minions because running taunt minions in excess results in lower average stat to cost efficiency. It does however have a fairly favourable matchup versus "all charges and stealths".

All Charges and Stealths
This deck runs every card that has charge or stealth as well as some class specific burn cards. Hunter and mage are most commonly seen running this deck. The idea is that if you play a charge card or a stealth card and attack the opponents face as soon as possible, there's nothing they can do about it. Throw in some burn and you might get lethal. This deck tends to crumple to all but the slowest decks due to not actually dealing much damage and having very vulnerable minions making it easy to take board advantage from them. If they have a favourable matchup it would be versus the all legendaries deck

All Legendaries
These guys tend to creep out around the start of a new season. I guess they want to show off to all the new players. You'll primarily find them in warrior and druid varieties. They like to run as many legendaries as possible with no regards to synergy and possibly a few synergistic class cards like innervate. Before TGT came out, this deck tended to lose often to any deck that could win by turn 6 as a result of having very little early plays. Slow decks like "all taunts" may easily fall victim to this deck.

Tempostorm ranks decks from 1 (best) to 24 (worst) as the following, with any decks that didn't make the list as worse than all of the ones listed.

1. Patron Warrior*
2. Secret Paladin*
3. Mech Mage*
4. Dragon Priest*

5. Mid-Range Druid*
6. Freeze Mage
7. Control Warrior
8. Handlock*
9. Aggro Paladin
10. Token Druid
11. Mid-Range Hunter*
12. Zoolock
13. Demon Handlock

14. Hybrid Hunter
15. Echo Mage
16. Tempo Mage
17. Face Hunter
18. Dragon Warrior
19. Oil Rogue*
20. Mid-Range Paladin

21. Totem Shaman*
22. Mech Rogue
23. Malygos Shaman
24. Lock and Load Hunter

The ones with a * are considered to be the top decks for each class. Rogue is the only class to have their top deck be tier 3, and shaman is the only class to have their top deck be tier 4. This list is effectively assembled based on how you do vs Patron Warrior, Secret Paladin, and Dragon Priest; which are effectively the meta-defining decks right now.

Since I have no stomach for Legendary grind, I spend most of the season bouncing between ranks 12 and 5.

In that range, if you see a [CLASS], it's likely to be...

Priest: Dragon is by far the most popular choice, with traditional Control a distant second.
Mage: Tempo is the most likely deck, followed by Mech and Freeze.
Warlock: Handlock. Zoo has somewhat fallen out of favor for now.
Rogue: Oil.
Hunter: An almost even split between face, midrange and hybrid.
Paladin: Secrets is the strongest and most popular deck of the week, although it's a toss-up whether you get an aggro version with Secretkeepers and 8-10 secrets or a midrangey one with 5-6 secrets and Boom/Tirion.
Warrior: Still Patron or Control. If the latter, expect to see Justicar Trueheart and have a plan in case the game goes to fatigue.
Shaman: Pretty rare now that totem craze has died down. Mech is less popular now. Expect either midrange stuff or OTK Malygos.
Druid: Call it ramp, call it combo, there's little difference between the two now. You see Malfurion pop up, you instantly know 90% of the deck.

Nobody expects the Natural Remedies Druid!

what are the tiers based on? why not split them into four sections of six?

Tempostorm's meta snapshot isn't perfect. Sometimes they have decks that seem weirdly out of place, or they have suboptimal lists. (I remember a few weeks back they had Hobgoblin Warlock on the list, despite it being bad and nobody ever playing it). But overall they do a decent enough job that it's a valuable source of information.

Tiers are mostly subject to the panel that ranks the decks.

Tiers are somewhat arbitray, but generally tier 1 is the top decks and decks that effectively hard counter them while doing well against a lot of other things. Tier 2 decks are decks that do well enough against tier 1 decks to still be good and do really well against other tier 2 decks, but they generally have a slightly bad matchup against more than half of the tier 1 decks. Tier 3 are "viable" decks where you are going to have a bit of trouble. And "tier 4" is an arbitrary cut-off created to **** about the state shaman and sometimes rogue are in.

Edit: Right now for example, tier 1 is patron, secret pally, and two decks that can do really well against at least secret pally without having a large number of bad matchups.

I think the important things to keep in mind in this meta are:
1-2 hp boards without deathrattles are easily punished by Patron, Control Warrior, and Combo Druid.
You need a plan to stop decks like Mech Mage, Dragon Priest, and Zoolock from establishing too big of a board position.
It's good to have an answer to Mechwarper and Darnassus Aspirant.
Warrior and Paladin both have top decks and both play 3-4 weapons. Throwing an Acidic Swamp Ooze or Harrison Jones in your deck could prove to be a good tech choice.
With Secret Paladin, Freeze Mage, and Tempo Mage all being very popular, secret tech like a Flare or Kezan Mystic might be a good choice, but both are pretty weak in the situation where the deck your facing doesn't include secrets.