I'm not as familiar with Marvel, but I'll try.
Spider-Man: It's actually really hard to beat the original Stan Lee run. He had a near record breaking run on the title with an uninterrupted run from 1-100, coming back to the title for a couple issues here and there, but he was more or less done with writing at that point. Truth be told, this is really the only occasion I like Stan Lee as a writer. I also really enjoy Brian Michael Bendis' run on Ultimate Spider-Man, one of the few to actually beat Stan Lee's run on the original Spider-Man. He's actually still writing it and he started in 2000. It was relaunched after issue 133, but now it's been set back to it's original numbering and he's now pulling around to issue 200, I think this is record breaking. But yeah, it puts Spider-Man in a modern setting. I haven't read it in years, but the first 11 or so trades are fantastic and I hear it keeps its quality up until the relaunch at 133.
Fantastic Four: John Byrne's run from the 80s was fantastic and by all accounts, it's definitive. He wrote from 209-221 and then from 232-293. I also am a huge fan of Mark Waid's run on the title. He wrote from #60 to #70 and then it was renumbered when it hit #500 and he continued to #524. I also hear great things about Jonathan Hickman's run from #570-588. It was relaunched as FF and I hear it's still good, but I'm a purist when it comes to the World's Greatest Comic Magazine.
Iron Man: Not really familiar with him outside of the movie.
Thor: Also not familiar with his comics. I do know that there's a lot quality runs of him through the years. Walt Simonson's run is from 337-382. That run is more or less considered the definitive Thor. I also hear great things about Dan Jurgen's run where he wrote from 1-79 (volume 2).
Hulk: Haha again, not really a fan. When I first got into comics, the Hulk was in the middle of a rather long and drawn out run. Apparently it started good, but just went too long. So I never got into it. I hear Peter David's (yes the Star Trek writer) run is the definitive Hulk. I couldn't find issue numbers but it was in the mid-80s.
Avengers: Literally there is something for everyone. It's the main headline book right now and there are four different titles. I liked Geoff Johns' short run from 57-76. There are some great trades out there like Avengers Under Siege and Avengers Forever, which are good. Kurt Busiek, who wrote the title from 1-56 (with a small break somewhere in the middle) also apparently was very good. I've read parts and I really liked them. Bendis' run who wrote after Johns is... controversial at best. I'd say read it. It may be for you. Certainly not for everyone.
Hercules: Don't know. He's more of a supporting character. Can't think of anywhere he's been featured off the top of my head. I think he had a solo series not too long ago.
Black Widow: Same as above. She's had a few solo series', but can't think of any stand outs.
X-Men: The group that got me into comics. The X-Men has two phases. Inspired and creatively bankrupt. Really nowhere in between. Right now, they are creatively bankrupt. My favorite run of theirs was Grant Morrison's (took me too long to mention him... haha) run from the early 2000s. He wrote a title called New X-Men which ran from 114-154. It's different, but I think hands down it's my favorite comic run of all time. There's also Chris Claremont's run of an astounding 16 years. 94-279 uninterrupted. It has its ups and downs but nearly every major thing in the X-Men mythos happened on his watch.
Doctor Strange: Not really sure. I always found him more interesting popping up from time to time in random books.