Marvel's A-list for nearly four decades have been Spider-Man, X-Men, Hulk and Fantastic Four. Regardless of what you think, that's the true A-list. Characters like Daredevil, Thor, Captain America and yes, Iron Man, occasionally managed to touch it, but make no mistake, they were always below them by some margin. It's only in recent times (I'm talking post 2000) that most of these characters really got their status upgraded properly, and with this expansion of the A-list they also upgraded a lot of C-listers to B-listers (Iron Fist and Luke Cage being the biggest examples).
People also need to keep in mind that The Avengers originally wasn't a team of Marvel's most popular heroes (like the Justice League was). In its early iterations it was Marvel's intent to popularize less popular characters and boost the sales of their solo books, and that's also one of the reason the roster changed often throughout its course so they could spotlight different (and new) characters. Once again, it wasn't until the post 2000 era that this was changed.