Huh... that's a tough question. Are you speaking like popularity wise? Because in North America I'd probably go with Jotaro, but in the show Joseph does show up in most parts and he's always a part of it (I haven't read part 4 in the manga yet). Although you could make the case it's Jonathan because it was his legendary fight with Dio that everybody knows about, and I always look at Part 1 as sort of a legacy arc, because a lot of people start off with Part 3 instead so they look at it more as like a part of the history and lore instead of just another part of the Anime.