I suspect, like most people who started playing RPGs in grade school, that there is no real contest for this question.
It's got to be ADVANCED DUNGEONS & DRAGONS.
We played AD&D a lot when I was a kid in grade school and in high school too. That's not necessarily because AD&D was the greatest game in the world, but it was pretty much the only RPG I was exposed to until high school. And we played it a lot because we were kids, and we may not have had a lot of money, but we had plenty of time to spend doing whatever we wanted. We would regularly play all day long, and on occasion play all weekend. Eight hour sessions were the rule, not the exception.
In high school, we found some other games that we liked too, including my beloved VILLAINS & VIGILANTES, TWILIGHT 2000, CYBERPUNK, TEENAGE MUTANT NINJA TURTLES AND OTHER STRANGENESS, and BEYOND THE SUPERNATURAL. All of them hold a special place in my heart, but we always came back to AD&D.
In university, it was pretty much second edition D&D and some smatterings of V&V, with summer expeditions into CALL OF CTHULHU.
More recently: We played a lot of D&D 3rd Edition when that was the new hotness, and I had a long multi-year FORGOTTEN REALMS campaign that I enjoyed quite a bit. We also played the hell out of MUTANTS & MASTERMINDS first edition, and SHADOWRUN. Later, we played a lot of CINEMATIC UNISYSTEM, FATE, and PRIMETIME ADVENTURES.
Recently, I've been reading through CASTLES & CRUSADES, and it hits a lot of sweet spots for me, despite the fact that dungeon crawls have kind of worn themselves out for me and most of my players. And if you asked me why, it's probably because it reminds me a lot of that game I played a lot when I was young and the summers were endless.