Baldur's Gate.
Yeah, I'd played the Ultimas about a decade before BG came out, and the Bard's Tales after that, and the Gold Box series, and the Darksun games, and the Menzoberranzans and Eye of the Beholders and Lands of Lores, but I'd spent most of my computer-user years playing those types of games exclusively and watching them evolve. They were my introduction into computing, and the Round 42s and Thexders and other arcadey games of that time were nothing more than a blip on the radar.
But then gaming died in the middle 90's for me, and the very first game that caught my eye after that long drought was Baldur's Gate; saw it on the shelf in Radio Shack and thought: "hey... A forgotten Realms game. They still making those?" And, based on nothing more than my recollections of the old gold box series, I picked it up on a whim; I was a little leery of the real time combat and so at first passed it over, but a few minutes later I was in EB and heard someone asking the counter-monkey to call around for a game that he'd been waiting for for ages called "Baldur's Gate", because no one in the city seemed to have any copies left in stock, so I went back to Radio Shack and picked it up, being the perverse individual that I am. Hadn't heard anything about it, had given up gaming in general for dead, but that game hit me like a ton of bricks when I first fired it up.
There have been better cRPGs, but that's the only one that made me sit right up and say "now THIS is a cRPG". More for the circumstances surrounding my first experience with the game than the game itself, perhaps, but such was the question.