Thanos wins this one easily. Much better fighter than Superman, and still way more powerful...even when Superman was sundipped he needed the Martian Manhunter's and Wonder Woman's help to move the Earth. Thanos could probably do it single-handedly. And Thanos has taken cosmic blasts without being affected at all...his durability is way above that of the Silver Surfer, who can also easily stand inside a sun. And of course you've forgotten that with Thanos's level of cosmic energy at his disposal, he could simply destroy the sun they're in (which would definitely hurt Superman, but Thanos could take it easily) or drain or extinguish it. Not to mention that he could also drain Superman's energy. And it's not like Superman ever fights at super-speed against enemies that aren't super-fast themselves (not that it would help against Thanos if he did). Superman can't beat Thanos EVER. No matter what.