I grew up on Star Trek (among other things) and was exactly the right age when Star Wars came along — 12. In 1977 there had never been anything like it, and those special effects, combined with a competent retelling of Joseph Campbell's hero's journey pretty much blew my mind. I watched it (I think) 15 times in the summer it was first released, going to the theatre again and again and again; I would put myself to sleep reciting (verbatim) every line of dialogue in the film.
So I guess ... Star Wars.
Now I'm 50, though, and both have become franchises almost entirely unrelated to their origins, I just want to see the third series of Orphan Black.