I assume you mean "In Sultai" given the Muldrotha comment, which takes away my two favorites (Nicol Bolas, Dragon-God and Tamiyo, Field Researcher). I think the best, roughly in order, are...
Ugin, the Spirit Dragon: Draw 7, gain 7, play 7 SHOULD just win the game on the spot.
Liliana, Dreadhorde General: One-sided Tragic Arrogance effect that also hits lands should completely cripple opponents to the point that winning is just a formality, and unlike most of these chumps she sticks around!
Jace, Architect of Thought: Super duper bribery on all opponents AND a free permanent for you to find any of these other 'walkers or game winning effects. Less good in this specific case compared to the Doubling Season case because if you pull a 'walker you won't necessarily get to ultimate again.
Jace, Unraveler of Secrets: This doesn't win the game, but Erayo, Soratami Ascendant is banned and this gets you that same effect. Your opponents might just scoop in disgust?
From there, there are several sort of "Tier 2" effects that aren't as powerful but are still game-winning especially in concert with a few other walkers.
Mu Yanling: Surprising, for sure, but "take an extra turn and tap all possible blockers" should be a powerful play if not necessarily a game-winning one.
Liliana Vess: Rise of the Dark Realms on a budget is fun, and she helps you assemble the comboes.
Kiora, Master of the Depths: 24 power and three fight triggers isn't broken or anything like some of these, but is a good way to end the game in concert with any of these other effects.
Vraska, Relic Seeker/Sorin Markov: Powerful effects but need some support to make it an instant win.
Teferi, Temporal Archmage: If you are going for a long, grindy game this emblem makes all your Planeswalkers super powerful and obnoxious.
Tamiyo, the Moon Sage and Will Kenrith are also usually pretty spicy, but less good in Muldrotha. Jace, the Mind Sculptor is also obviously very powerful but can't ultimate instantly without additional support.