I definitely don't think Superman is stronger than Thanos. Thor has feats that are definitely comparable to Supes, and he's in the same strength class as Supes. Savage Hulk, at his normal strength functioning strength, (before he gets pissed,) is about as strong as normal Superman. Prof Hulk has a functional strength just slightly below Savage Hulk's. Herc has insane feats of strength, (holding up the Earth, etc.) and he's easily in Supes class.
Thanos smacked around these three + Thing without trying. Even sundipped, I don't see Supes matching Thanos. Thanos can increase his strength to incalcuable propotions with cosmic power. Any way you put it, Thanos is stronger.