This could be quite a broad topic so first of all
What defines an A lister?
Dictionary.com defines an A-lister as:
A group of desirable or admired people who are welcomed especially in social and professional situations:
e.g.Hollywood's A list turned out for the Oscars.
In celebrity term it is usually the pull that celebrity has, or how Bankable they are.
This is measured on the Ulmer Scale named after James Ulmer.
The Ulmer scale is a 100-point method used to quantify a star's value to a
film production, in terms of getting a movie financed and the cameras
rolling. The Ulmer Scale also takes into account an actor's history
(box office successes vs. failures), versatility, professional
demeanor, and ability and willingness to travel and promote movies.
Bankability of Marvel Heroes
We can work out similar Bankability with the Marvel comic book heros if we can work out how many comics they have sold.
If we take DC's undisputed A-list as an example they have the Trinity of Batman, Superman and Wonderwoman, this is pretty universally accepted as their big hitters.
Lets try to find out Marvel's Trinity...
Now there are A LOT of comics that have been sold over the decades and I do not have a month to try to collect all these together and come up with an answer.
Using the figures provided on Comichron I have managed to get the figures from the 60s (1960-1969) and one year in the 90s (there was A LOT more figures from the 90s), 1995 to be precise.
I chose 1995 because it should be close enough to present day to give an idea on current popularity but be far enough removed from films that could skew the popularity.
Taking the 1960s as a whole the sales looked like this:
Title Avg Sales
Tales to Astonish/Incredible Hulk 2,173,925
Tales of Suspense/Captain America 2,103,794
Rawhide Kid 1,553,267
Amazing Spider-Man 1,447,473
Fantastic Four 1,344,143
Strange Tales/Doctor Strange 1,273,947
Sgt. Fury and his Howling Commandos 1,059,073
Kid Colt Outlaw 929,967
Journey into Mystery 757,727
This would give a Trinity of The Hulk, Captain America and Spiderman (ignoring Rawhide Kid for the fact we don't hear too much about him now)
The figures below are not pure sales but are the index attributed by the Diamond Comic Distributors. The higher the number the more sales.
Title Diamond Comic Distributors
Order Index (Combined for all Titles)
X-Men (and Spin offs) 10460.4
Spider-Man (and Spin offs) 3481.3
Fantastic Four (and Spin offs) 1129.54
The Hulk 557.58
Ghost Rider 370.55
Iron Man (and Spin offs) 360.08
Captain America 254.49
Doctor Strange 127.45
Guardians of the Galaxy 121.23
Blade the Vampire Hunter 34.8
The Trinity at this point has shifted a little with X-Men being undisputed kings, with Spider-Man and the Fantastic Four completing the set but quite a way behind.
I don't think it is a coincidence that these three were sold off when Marvel needed money to stop them from going bankrupt.
So Spider-Man and the Fantastic Four have been around the top for quite a while, whereas X-Men had a massive resurgence and most of the MCU characters had suffered a dramatic drop off pre-MCU.
Iron Man is quite a way down on the list from the 90s, as are Captain America and Thor. The Hulk Sits in fourth but his film rights were also sold off as Marvel needed the cash.