Here's my bet, Dormammu vs Thanos (without any of the infinity stones)
would not even be close, Dormammu would utterly wipe out Thanos without too much trouble.
For that matter I doubt Odin or any other character we've seen so far in the MCU could stand against Dormammu one on one.
Thanos with one infinity stone, or maybe 2 would be a decent battle - I mean
Dormammu appeared to pretty much have all the power of the Dark Dimension at his command (so he had the power of an entire universe at his beck and call, a really weird, trippy, messed up universe, but suffice it to say he was a godlike cosmic entity).
So, If the battle took place inside the Dark Dimension, I would still give it to Dormammu.
Thanos + the infinity gauntlet and all six stones..... well I think he'd have enough power to destroy Dormammu, and the Dark Dimension, I mean doesn't that pretty much make him completely omnipotent (not just a god, but actually God) with power over all time, space, matter, energy and psychic stuff?
As for Dr Strange, and how he "defeated" Dormammu - well "defeated" doesn't really accurately describe what happened.
Strange really forced Dormammu into a stalemate by using the time gem (Eye of Agomotto) to create a time loop (which reset itself every time Dormammu KILLED Strange, which looked pretty effortless really). So he used an infinity stone plus a clever stratagem to force a draw, which Dormammu pretty much withdrew due to boredom and frustration.
Also, I suspect that the time stone was possibly the only one which could actually defeat Dormammu, because, as Strange said he " brought time with him." when he took the Eye into the Dark Dimension (because time doesn't exist there). So the Eye had a power that Dormammu was unprepared for.
I liked that a lot, that Dormammu was so powerful that Strange could never hope to challenge him, but instead had to rely on his wits.