I should name some old monster from the Atlas brand horror comics which predated The Fantastic Four and claim my geek cred. (Sidenote: Groot's first appearance came from this era.) I really Black Panther. I have to, however, go with the obvious choice.
Spider-Man: I choose the wall crawler because of the behind-the-scenes origin of the character. In interviews, Stan explained that he specifically created Peter Parker as a rejection of the tropes that dominated early superhero comic books. Superheroes were not supposed to have real world problems like rocky relationships or school tests. The place for teenagers was as the plucky sidekick of the superhero. Stan hated these two rules and directly contradicted both by co-creating (with Jack Kirby) a teenaged superhero in nobody elses shadow who had his own teenage problems in addition to the villain of the month.
My favorite Stan Lee cameo was in Iron Man when he was mistaken for Hugh Hefner.
I think The Onion said it best: "Stan Lee Creator of Beloved Marvel Character Stan Lee Dies."