My first was D&D (Holmes basic, not that I identified it as such for years) but I largely switched to Runequest within a month. The more-or-less local club had these huge (20+ players, with about 10-12 per table most nights) open campaigns running RQ and later Traveller for anyone who was interested and could demonstrate some degree of maturity. Usually one of the two or three tables was adults-only while the other were mixed. I remember being extremely happy when my 13-year old self "graduated" to the adults table on the basis of "not being insufferable" according to the older guys who vouched for me.
Since they seem to be coming up so often on this thread and the genre is a particular interest of mine, my first supers games were V&V and Champions, both first edition and in that order. Unlike many supers fans I have never actually played TSR's Marvel rules, in large part due to disinterest in roleplaying in either of the Big Two's published settings. Not my gig, I want to create my own characters and world.