Spider-Man - I have a book which is called "The Best of Spider-Man" and in it, it reprints a bunch of strips from the newspapers. It has half a dozen stories, and I love it dearly. However, it's a series of random stories, so it's not eligible. As a 'single story' or 'event', I'd have to go with... "Learning Curve" (Ultimate Spider-Man 8-13). It was the first real Ultimate trade I got, and it was an awesome single, Spider-Man story that held itself for 6 issues. Superb. I don't think USM ever did as good as that arc. There's a number of other Spidey stories I love: the two issues of TANGLED WEB that Darwyn Cooke did, as well as the Rucka/Risso "Severance Package" (which isn't really a Spider-Man story), and the "Flowers for Rhino" two-parter. I love Peter Parker: Spider-Man #37 "Snow Day" too. And then there's SPIDER-MAN LOVES MARY JANE. But I think the best I've read is Learning Curve. Bendis did a stellar job on it.
Captain America - Outside of THE ULTIMATES or EARTH X, I can't think of a solo Captain America story I've read/liked.
Fantastic Four - My favourite is Warren Ellis' "N-Zone" (Ultimate Fantastic Four #13-18) primarily because it's the first time someone had the F4 go somewhere odd and spend a whole issue just having them gawp at the sheer otherworldliness of it all. The N-Zone felt like an alien world, not just some weird place where Hobbits lived. I also liked Ellis' Doom arc before it, and FANTASTIC FOUR: 1 2 3 4 by Grant Morrison and Jae Lee. But again, not a big F4 guy.
Avengers - THE ULTIMATES. Specifically #1-6 "Superhuman". Close send is THE ULTIMATES 2 #1-6 "Gods and Monsters". Just no question.
X-Men - While Grant Morrison's NEW X-MEN was awesome, it was a bit all over the place, and as awesome as Mark Millar's "The Tomorrow People" was, I have to give it to Joss Whedon and John Cassaday's ASTONISHING X-MEN. It was wonderful fun.
Hulk - Nope. No solo Hulk stories that I can think of...
Cosmic - If the EARTH X trilogy counts, I pick that.
B-lister - Ennis' Punisher MAX is not only the best 'b-lister' Marvel comic I can think of... it's one of the best comics I've ever read. It's superb.
Ultimate - I know I picked it, but it's THE ULTIMATES.
'Line-wide' - EARTH X trilogy is the best crossover I've ever read. It's vast, effective, intelligent, and it should be canon.
I feel like I've forgotten something.