Unless Thanos gets some Infinity Stones, I am backing this guy:
He just has way too many options. He can teleport Thanos to space with just a wave of his hand, for example. Or blink him out of existence. Or phase him into his surroundings. Or pull his mind into the astral plane, leaving his body vacant etc.
With the IG, it basically becomes who can hit the other one first, though I feel like Thanos might win this one, due to more combat experience in that kind of situation.
Well, he was able to eventually counteract the Time Demon's abilities when confronting them (even the Shadow King had to confront them in that other weird picture realm to challenge them directly), but I am not sure what he actually did there. And there was also that time in S1 where Shadow King-possessed David froze time (the timeline was confirmed to be frozen by Cary). He appears unable to travel forwards or backwards to other times though.
I don't think anyone ever actually tried though. I wouldn't be surprised if it was possible though, considering someone could rewind time on a large scale while using it, like Strange did at the end of his solo movie. And that is kind of the same effect, through a different mechanism, as he essentially brought himself back to an earlier point in time, but changed time instead of himself to do so.
Well, they had to involve a degree of PIS, or he could have easily solo'd everyone with just the Reality Stone. Kind of hard to fight when you're spaghetti.
Which is also what makes characters like Legion so dangerous. They don't really shoot beams or things that you can block. They just use their powers on you directly.