The bare minimum you need for a Marvel action figure collection is one each of the following:
Spider-Man
Captain America
Thor
Iron Man
Hulk
Thing (to represent the FF)
Wolverine (to represent the X-Men)
Thanos (one heavy-hitter to represent bad guys)
Bare minimum.
It gets unwieldy very quickly. When I rebuilt my collection from scratch 15 years ago, I tried this exercise. I thought, "I love Marvel. What would a small, complete Marvel collection look like?" It becomes
If You Give a Mouse a Cookie.
You
need Spider-Man. What's Spider-Man without Green Goblin? Green Goblin without Doc Ock? Doc Ock without the Sinister Six? Spidey without allies to help him defeat the Sinister Six? And on and on.
You
need Wolverine. Wolverine needs the X-Men. Most addicts here would agree that any good X-Men collection needs Beast, Colossus, Cyclops, Gambit, Iceman, Jean, Kitty, Nightcrawler, Rogue, and Storm at a minimum. Plus Magneto for good measure.
You
need Hulk. He needs Cap, Iron Man, and Thor. They need Ultron, Loki, Red Skull, and Thanos to beat up on. Honestly, I probably could've stopped there with the Avengers, but I was already dedicating a shelf to Hulk/Cap/IM/Thor, so I fleshed it out with the classic movie Avengers + BP, Doc Strange, Scarlet Witch, and Vision.
Then big characters like Doctor Doom. How can you have a Doom without a Fantastic Four?
I tried capping myself at 50 Marvel characters. I couldn't do it. I'm at 70 and I'm still missing a lot of characters folks would consider essential. I don't have Ghost Rider or almost any cosmic characters. I wound up with my essentials + some non-essential favorites (Squirrel Girl, Beta Ray Bill), which I guess is the healthiest way to do it.
Spider-Man
Captain America
The Falcon
Hulk
Iron Man
Thor
The Fantastic Four
Green Goblin
The Lizard
Falcon but no X-Men? Avengers fans have gone too far.
And yeah, very dude-heavy, but Marvel's never done a great job of fully establishing the "even my mom recognizes her" marquee female character, and a ton of their most iconic women are behind the X-Men label which has its own kinda siloing happening.
It's Storm. I don't really think it's close, either. She's an icon. Everyone on Earth wants to be with Storm, but we all know she's too good for us.