Round 1 (Ultron with the Mind Stone): Thanos is way above Ultron in stats, and we have no idea how Ultron would use the Mind Stone or if he even could, so Thanos would stomp here.
Round 2 (Age of Ultron Avengers): The biggest threats here are Thor, Hulk, Vision, and Wanda, though each of them are less powerful here than they are in the future. Together all of the Avengers may win, but Thanos may also have a chance. I can’t decide.
Round 3 (The Ancient One): Thanos’ abilities without the Stones is 100% melee. His skillset and gear is not suited for combating magic based powers that he has no counter to, which is why I think Scarlet Witch did so well against him. Not only were her telekinetic abilities very strong, but they were pretty much perfectly suited for countering Thanos, keeping him away from her and suspending him such that Thanos couldn’t do anything as she started tearing him apart. Canonically, the Ancient One would have many of the same abilities Doctor Strange has shown and at a similar level, which should give her many options to defeat Thanos, but her lack of on screen feats hurts her here, so I would go with Thanos.
Round 4 (Captain Marvel): It seemed pretty clear to me on first viewing that the Russos wanted Danvers to be roughly equal or even superior to Thanos without the stones. She got some very impressive feats in Endgame, like traveling a thousand light years in no more than a day while carrying a large spaceship, destroying Thanos’ mothership in two bullrushes, and physically tangling with Thanos and even tanking a headbutt from him. It took Thanos popping the Power Stone from the gauntlet to empower his punch to take her out. Without the stones to help him, Captain Marvel is physically comparable or even superior to Thanos and has very fast flight and energy blasts, which Thanos doesn’t. For me, Captain Marvel would take the solid majority.
Round 5 (Doctor Strange): The Doc would wipe the floor with Thanos Scarlet Witch style. He has so many options to take Thanos out that Thanos can’t counter, he’s like a much more versatile Scarlet Witch.
Round 6 (Prime Thor): Thor is a character that Thanos can face better than Scarlet Witch or Doctor Strange. His raw power is comparable and arguably superior to their’s, but Thor's abilities are much more physical. An obese, drunken Thor who had done nothing but play Fortnite for five years did a lot better than I expected against Thanos. He got into multiple grapples with Thanos and held his own, and was heavily resisting Thanos pushing Stormbreaker into his chest when Thanos had MASSIVE leverage over Thor. Thor from Infinity War would at the very least be equal to Thanos physically, and in my mind, be physically superior to him like Captain Marvel likely was. Thor was the star of Infinity War and single handedly turned the tide of the entire movie. He needed to be nerfed in Endgame so that the other characters could shine and get their proper farewells. Prime Thor would take the easy majority over a base Thanos.
In fact, all of the Avengers’ heavy hitters were taken out by Thanos with external amps or were incapacitated in some manner. Thanos forced Scarlet Witch to let him go and knocked her out with the mothership barrage, Carol was only taken out because of the Power Stone, Strange was occupied holding back the massive amount of water telekinetically, Thor was very out of shape, Vision was still dead, and Hulk was heavily wounded after he brought back half the universe with the Stones. All their most powerful assets were basically out of commission in that fight in order to close out the Infinity Saga like it started: with Iron Man.