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.

--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.

--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.