As far as definitive voices go...
Wolverine/Logan :: Cal Dodd
When a guy manages to come up with two different takes that fit the character so darn well....you know he's the guy. He gave that Clint Eastwood-ish tone in the X-Men Animated Series, and in a game, he took a more darker turn to the voice that would be appropriate for the comics. So yeah...he's awesome. I like Scott McNeil and Steve Blum though.
Spider-Man/Peter Parker :: Josh Keaton
I've always enjoyed his Spidey. Keaton's Spidey is funny, sympathetic, intelligent and can sound pretty dangerous at times. His performance is very natural and he never goes too over-the-top. So yeah, he'll always be Spidey to me.
Nightcrawler :: Liam O'Brien
He is Nightcrawler. 'nuff said.
Magneto :: David Hemblen
A lot of voices this character gets make him more villainous than he really is. Hemblen gives him a voice that you would understand why people fear up and why mutants everywhere look up to him.
Dr. Doom :: Lex Lang
Boy...this guy has RANGE. Seriously, I remember the days where he used his normal voice for everything, and now he pulls voices like this out of nowhere. That said, the moment he said that line to Wasp, I immediately thought in my head, "Yeah...THAT'S Doom. Right there." He has the perfect amount of menace and arrogance that Doom needs.
There's definitely more, but those just come to mind.
"Don't take life too seriously; you'll never make it out alive!"
- Bugs Bunny, "Rabbit Feat" (1960)