If I recall, there was that one Gambit plot line- an alternate reality version was going around, trying to kill all of his equivalents because they almost always end up accidentally destroying Earth. So, I'd say that Gambit is a contender.
Still, Wolverine can't die. So, that's a plus for him. If Shadowcat didn't have that adamantium weakness, she'd also be pretty much top tier. Quicksilver is also pretty overpowered, just like the DC speedsters- he managed to bite close to every superhero on the planet in minutes, or seconds, in Marvel Zombies.
Hulk once ripped Wolverine apart. That said, Wolverine was like "Whatever, man" and was fine, so that's all good. Oh, oh, and Deadpool has that plot shield
Actually... That reminds me of Squirrel Girl, who I think is canonically the most powerful of all.
No idea about the second question, though. I personally like most all of Marvel's characters.