Secret of Mana - My first RPG. If that ain't defining, I dunno what is

Deus Ex - On top of showing me what a branching narrative, where your choices matter, can do in games, it also destroyed my view of what is and isn't an RPG. Before I played Deus Ex, I'd have laughed at the concept of an RPG that looks like a first-person shooter.

Ar Tonelico - Not just the genre, but games as a whole really, this is the game (and series) that propelled music in games forward a lot for me. Sure, before playing this I thought theme X from game Y was cool and all, but it became more integral to the experience for me after this game. I also started paying a lot more attention to context, subtext and such in game music.

I'm not sure I can think of two others besides those. Some of them I'd like to mention but it'd be a stretch to say they actually changed my views on the genre.