Who or what, in your opinion, should be the Final Boss of Magic: the Gathering ?

Yes, this isn't a very serious thread, and it's based on a meme. Still, what do you think ?

Final Boss was Yawgmoth. Nothing after the Weatherlight Saga has been as strongly "The story of Magic: The Gathering" with higher stakes or a bigger boss. What began in Alpha ended in Apocalypse; the later blocks are all Gaiden games and now the much-reduced sequel in the form of the Gatewatch.

Yeah I love Nicol Bolas but when you look at the whole of magic story his story is like a sidequest or DLC pack

If I may, in order of 'Final Bosses'

--Yawgmoth, the manifestation of Black Mana. Yawgmoth is easily the most powerful entity ever featured in the M:tG story line. Just to prove my point, look at his match up against Urza in the card 'Corrupt'. Urza, who many would argue is the most powerful Planeswalker, get's literally smacked up without much of a thought. He even managed to convert his biggest enemy, Urza to fight along his ranks, and his multiplanar Phyrexian empire is like nothing currently in the story. Hell he even has 'bosses' under him, including Volrath, Crovax, and Mishra. Even the new Phyrexians worship him, even if they don't know his name.
--After Yawgmoth, Leshrac Nightwalker. The Walker of the Night was a planeswalker defeated by no other then Jodah, and was trapped in the moving plane of Shandalar. An ambitious and cunning walker of his own accord, in his millennial life he allied w/ the likes of Lim-Dul (the Necromancer), Tevest Szat (the Doom of Fools, and one of Urza's Nine Titans), and the Myojin of Nights Reach. While he may have been defeated by Nicol Bolas, that doesn't change the fact he remains one of the most cunning and powerful villains to ever take the helm, especially fans of the 90's Battlemage/Shandalar games.
--The three Eldrazi Titans. There is something about creatures coming from the Vast Eternities inbetween the planes that hits a chord so well. They eat. They don't have ulterior motives, they don't scheme. They eat, it's what they do, and they do it so well. Though Emrakul's death was retconned (for better or for worse), it was nice to have a concept of the Eldrazi being so powerful even Planeswalkers were little more then speed bumps to them.
--Nicol Bolas: Yeah, he's something of a Gary Stu, but he's the last remaining Elder Dragon. Bolas has been carved out to being the best Grixis has to offer, in terms of power in the story, and in terms of design philosophy. Bolas is arrogant, cunning, and ruthless, making him the perfect enemy for the rag tag group of heroes. While in his own time, he wasn't that powerful of a walker, the mending has changed that, making him more then likely the most powerful force in the multiverse, even commanding lesser Planeswalkers under him (something only one other character on this list can claim). While seeing him all the time gets tiresome (it does), I'm a fan of Galactus style characters, and in the current lore of MtG, Bolas gets that well.
--Baron Sengir: Probably the most insignificant character on this list is easily one of the most well built. Trapped in Ulgrotha after a Planeswalker summoned him (and then died), Baron Sengir set out to carve his new home in his own image. While not well versed in Magic, he's is knowledgeable. He has struck deals across Ulgrotha, and made himself a 'Barony' and a 'Family' that includes a grandmother (who in turns is a dementia riddled Planeswalker, and who wears an apocalypse causing chime around her neck), a felled dwarf Princess, an fallen Knight, and a cut throat. Patient and cunning, but loyal and fair, the Baron is easily one of the best designed 'Bosses' WotC has ever producted. He may currently be under rule of Bolas.
--Ob Nixilis: The demon planeswalker, Ob killed all the denizen of his plane, and then continued to do so, until becoming trapped on Zendikar. He's a less interesting version of Tevest Szat and Leshrac, but he's the best the current design team has to offer.
--Ravidel: While lower then I'd like, Ravidel is never mentioned on any cards, nor is he depicted, making him hard to put onto the list. Ravindel, the Battlemage, was easily the most pimp walker to ever exist. Flying on a planeswalking barge (made out of the Elder Dragon Chromium), having an orc army armed with Rocket Launchers, mastering all five colors of magic, and owning all the Moxes (and a Chaos Orb), Ravidel was everything a pre-revisionist walker could hope to be, in both larger then life flare, unparalleled insanity, and vengeful cunning. His plan for power was literally to create a giant Planeswalker war, and then kill the survivors. It's a rule of thumb that any pre-revisionist planeswalkers that don't appear later in the story simply died in this, and it's well known that Jared Carthalion, the Shadow Mage.
Geyadrone Dihada: The lessest of Boss walkers, was none other then this walker, who's most notable achievement is creating Dakkon Blackblade, and having that Planeswalker (and a number of other assorted bad asses) work under her. She ironically is mentioned on a card, the 5th edition print of Lure, just in time for the continuity change.

Honorable Mentions, Taysir, Kaervek, Mishra, Korona, Ixidor, The Cabal.

Arguable on a few points:
He's not the manifestation of black mana. He's the artificial soul of an entire world with a core identity of black.
Aaaaand New Phyrexia doesn't worship him. He failed. In the book of Phyrexia, that makes you weak and worth contempt.

You're vastly underestimating Bolas abilities pre-mending.
He was so powerful and vast he largely had to exist outside worlds and moved the eternities around himself rather than through them.
In battling other walkers, he rarely had a significant challenge, so he very much was an immensely powerful walker. Likely the MOST powerful walker, even beyond Urza and Taysir.

that's... possibly debatable. He might have appeared on surge of strength.
I swear that character is someone, but other than Ravidel, I can't place who it might be.

Shelkin Brownie. He breaks the team up.

I don't know about that, I don't remember him being that powerful in the Legends books.

Plus I'm pretty sure Yawgmoth could take him, and so could Urza (Taysir has always been a chump compared to those two).

Fun Fact: I met Ruth Thompson some years ago at Gen Con. Instead of having a booth in the artist alley, she had a small independent booth elsewhere in the hall. I remembered I had one of those on me, and asked who the character was, since he also appears on Justice and Dystopia.

She said it was just an original character she had designed, because in those days you were free to do what you want. She was hoping if she put him on enough cards, he could become a character, but it never happened.

Sarkhan Vol, Master of Time, Space and Dragons.

I actually would rather MTG move away from the BBEG model. It makes a story seem... small. Especially when there is literal infinity to work with in MTG.

I want to see a new enemy that can give Bolas a real run for his money, but is equally evil. I was hoping that Ob Nix would be that, but doesn't seem to be the case.

Bolas, or the Gatewatch post-Bolas, are afraid of something. In Bolas's case, it would be neo-walkers that could threaten his powerbase. In the Gatewatch's case, it could be something that they foresee as a threat on down the line. Either way, Bolas or the Gatewatch depending on your plot, could take preemptive steps against what they perceive as a threat, creating the Final Boss of the Gatewatch Arc in the process (as collateral damage or something).

Yeah, global conflicts > solo villains. (imho, obviously)

Lawyers....

Magic's core appeal is supposed to be "look at all these exciting and varied fantasy worlds!" so I guess a Final Boss would be something that would make them all the same. Phyrexia is sort of this, but a true Final Boss would probably have a faster process than them because "returning to Phyrexianised Planes" sounds really cool. It doesn't feel like a proper Final Boss to me if people are excited to buy the sets where they succeed, which is probably a flaw with all the current Magic villains. I know I read on a forum somewhere Bolas planned to take over Ravnica, and my first thought was "That sounds cool; I hope he succeeds!"

Tilt.