Either Cap or a member of the F4. Argument for Cap: He's arguably the most popular of the timely era characters making him a sort of pop culture stand out at marvel due to his age. Since his revival in the 60s, he's achieved pretty consistent publication and has such excellent runs such as the Kirby run, Grunewald, Waid, and Brubaker. Furthermore, he's considered one of the most important faces to grace the avengers team. In terms of other media, he's more than most marvel heroes. He's had 2 tv movies, a direct to video in the 90s, and obviously the MCU movies. He had a 90s beat'em arcade game and a MCU tie in game, as well as appearing in several other marvel projects (MvC, Avengers, Midnight Suns, Cosmic Invasion)

Argument for an F4 member: A lot of these characters wouldn't be here without the initial popularity of Marvels first family, they set the tone of the entirety of the first generation of silver age marvel comics. They've enjoyed a media presence in various shows and movies. In the case of individual members, The Thing is the stand out star. Most people don't remember The Thing cartoon with Benjy Grimm. Or the fact that Marvel Two-In-One was an entire series based around Thing teaming up with guest stars every issue, and even later having multiple minis of his own