I don't think I ever weighed in on this, so I'll do so now.
From sheer popularity and modern era recognizability, Spider-Man & Wolverine have to be in two slots of a Marvel "Big Three". True, they haven't been around as long as DC's World's Finest, but they've been the biggest comic and media stars for the past generation or so. The fact that Wolverine is stepping back from hogging everything the last 2-3 years didn't eliminate his rise in the 90's. And Spider-Man has always been Marvel's ironic character, and the movies just cemented it.
The trouble comes for me in choosing who completes that "Big Three" for Marvel. In terms of longevity, Capt. America should get it. He's one of few characters Marvel has who has endured for over 60 years and has always maintained some sort of ongoing, something Namor can't claim. The news of his death was covered aggressively by the media, almost half the attention they gave Spider-Man's Unmasking, or the death of Superman.
But vying for that third spot, on sheer popularity and recognizablity is The Hulk. The 70's show got him the media fame and the kids like him a lot more than they ever will like Capt. America. Between a new movie and WWH, the Jade Giant will be having a good year.
But for me, Cap completes the Trinity as an icon.
In a way it is a shame that Marvel doesn't have a heroine in their trinity as DC has WW, but then again, Wonder Woman's always been proven to be behind the first two capes in terms of DC's priority. Marvel hasn't had too many longstanding heroines who weren't part of teams. And those who were on teams are even few and far between to find some who have been around beyond 30 years. Wasp is there, but she's always been attached to Pym. Same deal with Invisible Woman, attached to Reed. Then there was Scarlet Witch, who had extended periods away from her twin, but Bendis has utterly ruined her almost beyond repair (turning a longtime Avenger heroine who overcame wonky powers and bad heritages to follow her destiny into an emotionally fragile & insane ****; for me, THAT was far more misogynist than a softcore hentai cover for HFH).
But, put me in the Spider-Man, Wolverine, and Capt. America camp.