Visually, I tend to think of iconic Marvel as the John Romita Sr. House style used in the 70's-80's. It wasn't only in the books, as both merchandise and cartoon series seemed to ape this look.
The X-Men I picture first are usually either the "All New All Different" team or the Jim Lee Era of Blue and Gold. Emma Frost is the Hellfire Club version.
The FF is the classic team, but I also still think of She-Hulk as a member and the Four Freedoms Plaza as their headquarters.
Avengers is usually some version of the team under Roy Thomas. (Hawkeye is in his classic purple, Iron Man is the red and gold model 4) Cap, Thor, Wasp, Scarlet Witch and Black Panther are also members.
Spider-man is his classic Romita red and blues (although I will always have a personal preference of the black costume) but Mary Jane is the big hair 90's look of Erik Larsen.
Hulk is his green "dumb" version. (Although, I personally prefer the Grey Mr. Fixit version.)