Thanos...unless Doom has something up his sleeve, (and he usually does), but that requires prep. A lot of prep against a guy like Thanos. One on one in an impromptu fight, it's a mismatch...though Doom's armour is no joke, and can withstand an insane amount of punishment. Probably able to stand up to whatever Thanos can dish out against him in terms of energy blasts, but offensively Doom is way out of his league. He would probably eventually flee.
Magneto can't really hurt Thanos, either, and would flee as well...unless he's killed.
I don't really know much about Carnage except that if Sentry was able to rip him apart, then Thanos could do it quicker and better.