Tough one. I've always liked Doc Ock: he's unquestionably one of Spidey's biggest villains and perhaps I've been brainwashed from Spider-Man:TAS and the comics that were based off that show, but I feel that he's one of the greatest Marvel villains. Martian Manhunter just isn't an iconic hero but his character is great. When Justice League started airing, he automatically became one of the best team members (as far as I'm concerned), along with Batman and Wonder Woman, thanks to stories like "A Knight of Shadows" and "Comfort and Joy". I think that as time went on, he became eclipsed by better developed characters like Hawkgirl and Green Lantern, but there's more to him than there is to Doc Ock. Really hard choice but I'll go with Martian Manhunter, because depth gets you further than being a character who has recognition, for me.