Rolemaster - First RPG, first love, I dug pretty deep in this one. Cut my teeth on early versions of excel thanks to this game.
Planescape - A setting rather than a RPG proper but still, i fell
hard for it. Mostly, Toni DiTerlizzi.
Werewolf: The Apocalypse - I played V:tM a lot but I
loved the themes, aesthetics, and artistic signature (more DiTerlizzi!) of W:tA. I read everything there was to read, played with concepts, made characters, plotted a potential campaign... but I never played
Ars Magica - Lots of good stuff! Loved the medieval paradigm, loved the system, setting books were amazing. I just couldn't find a proper group to play it regularly.
The One Ring - LotR is what brought me into gaming in the first place but for the first time, here was a RPG that actually managed to capture the elusive "Tolkien feel" (Sorry MERP, you were great game but not
quite Tolkien enough). Loved (and still love) many of its concepts and systems and was kind of an eye opener in RPG design.