I would have to go with Thanos on this one. The first Cube Doom got, all he wanted was his memory back. Doom likes to do things on his own. I could picture Thanos wishing for ultimate power. Doom would probably wish for something that would help him achieve ultimate power, but not give it to him outright.
Doom has given up power and walked away from it because he felt the win was cheap. I can't see doom wishing for ultimate power just for the sake of ultimate power. Thanos on the other hand I can see doing it.
So I would say Thanos on this one.