Probably Thanos. Korvac was meh in that arc. Out of all the cosmics in that issue :
Galactus died because he was nullified
Korvac beat Stranger fair and square
Korvac struck down Gardener when he was distracted
Korvac struck down the IB when he was distracted
Order and Chaos weren't really in it
LT ran
Shaper joined the cause
Grandmaster went to talk and got struck down
Korvac absorbed the Celestials on Earth
Now comes the idiocy. After all that, he still couldn't take over the universe "I maybe the most powerful being in the universe but I'm not more powerful than EVERY being in the universe". Thanos with a Cube took over the entire universe and that was 616 and canon.
Yet he still couldn't conquer the universe, in his own words "I may be the most powerful being in the universe but I'm not more powerful than every being in the universe". Then he whipped out the UN and offed himself and the rest of that alternate reality, with Death laughing all the way to the bank.
Thanos with Cube conquered 616 reality. He should stomp What If Korvac.
See thats one of that comics inconsistencies; Korvac scared off the LT, but he couldnt beat a manifestation of Eternity (that is my take on all the universes races comings after Korvac at once)...
Yup scared off the LT but couldn't destroy a universe under his own power or conquer it without using the UN. Also Death played him for a fool and beat him in the end. I'm going with Thanos with Cube for the win.