Of course Superman.
But name the number of times that a speed edge was decisive in a battle between at least somewhat evenly matched top tier bricks? Even harder to find are examples of a top tier brick using speed to beat a more powerful brick that the less powerful one would normally have lost to. (When a character is going to dominate another due to being decisively more powerful overall, writers often get creatively realistic with power sets as a way to make a lopsided fight interesting, but you rarely see a writer 'spoil' an otherwise balanced or interesting fight with some kind of insta-win power set usage, regardless of how realistic it would be).
Superman is faster in every way. The chances of this mattering in a fight vs. Thanos are low.
Superman would be favored, even at typical power levels, if no serious magic weakness came into play.
However, one consistent pattern with the magic weakness is that when you actually cast spells, rather than just use magic-based power (especially divine magic), then the magic weakness almost always comes into play to some extent.
So I'm typically favoring Magik, but not overwhelmingly as many Superman portrayals have Supes as a very crafty and resourceful dude.
However, there are quite a few high-end (including 'all out'😉 portrayals of Superman that might steamroller her, even in Limbo, even with the magic weakness.