1) Avengers
Best: Roy Thomas, with Busiek a close second
Worst: Chuck Austen in terms of worst quality, Bendis due to long-term damage to the book. Aaron a close third.
2) Iron Man
Haven't read any run aside from the original Stan Lee run and the Extremis arc by Warren Ellis.
3) Thor
Best: Walt Simonson
Worst: Jason Aaron
4) Captain America
Best: Mark Gruenwald
Worst: Haven't read a Cap run that I didn't like. Spencer's and Coates' looked like crap, but I never read them.
5) Black Widow
Have not read any of her runs
6) Hulk
Best: Al Ewing (I have not read PAD's run, aside from a couple of issues here and there)