When Thanos went up against Odin, he was knocked around like a ragdoll, despite wielding the Reality Gem.
1. He wasn't wielding the Reality Gem at all in that fight. I don't believe there's any strong evidence that he even had it with him at that time, though I could be wrong.
2. Thanos is much smarter and craftier than any other combantant listed(the only one close being Darkseid). He wouldn't just dive right into a brawl to the death with these guys, he'd turn them against eachother and have them whittle themselves down before he closed in to fight the last few left.
3. If Champion has the Power Gem, then Thanos would obviously make retrieving it his topmost priority as soon as the fight begins. With the Power Gem Thanos could easily defeat the majority of those listed.
Hmm I don't know, Ion should win this. The only time we have seen the full extent of the ION entity was with Kyle Rayner, which showed a power far beyond any being in this category.
Again, this was was Kyle Rayner + Ion + all of the power of the Central Power Battery that Hal Jordan as Parallax had left.
Ion itself doesn't not grant that ridiculous level of power. Always thinking of Ion at this power level is like always thinking of Thanos with the Infinity Gauntlet.
In regards to the fight itself: if it is simply a gigantic brawl, I think I'd go with Thor having the highest chance of victory closely followed by Monarch and Superboy Prime.