Dr Strange (What if) vs Thanos w/ Stones
Thanos with Power, Space, Soul, Reality and Time Stones.
Thanos as he appeared on Titan except now he has the Time Stone to even things up as Strange also has the Time Stone.
This is the Dr Strange that absorbed Several magical creatures. The same one that fought Ultron.
I'd back Strange Supreme. He carried his team against an Ultron who not only had all the Infinity Stones but, outside of the snap (which Thanos can't do here anyway), had much better feats using them as well. IMO, Strange Supreme is the most powerful hero throughout the entire MCU multiverse that we've gotten to date and by some margin too. Next would probably be What If Captain Marvel and Chad Thor, but Carol was killed by a lesser attack to the one that Strange literally ate.