Well I figure popularity comes and goes but there have been a few mainstays but a few characters have not only been popular but have spun off a number of characters. Either by publishers ripping them off or within the context of the stories.
Spiderman Siderwomen, Scarlett spider, 2099
Hulk She Hulks Sampson, Rulk, Skar
Cap other caps, US Agent, Falcap, Bucky Cap, various Nomad
Wolverine Daken, X-23, the whole weapon x thing
Thor Beta ray, Eric, Val, She Thor
Hank Pym Ant men, Wasp, Goliaths, Giant Men, Yellowjackets
Tough to pick a 7th Iron man has a couple, Thing has had a few She things. Been a few Ghost Riders.
Like I posted not based on popularity but on impact across the comics.