I went with Baldur's Gate. I've only played Ultima 7 (and the Underworlds) and a little bit of Wizardry 1 (yes, 1. Why I haven't tried the more modern ones is totally beyond me) otherwise there's a good chance I'd be voting for one of those.
I also really love the Elder Scrolls series, the Fallouts, and KOTOR, but Baldur's Gate struck a perfect balance with everything it did. It wasn't too linear or too open, it wasn't too story driven or too hack and slashy, it had solid characters and a decent plot, and the combat somehow managed to be simultaneously deep, accessible and fun.
If we were going for a single game it would have to be Planescape: Torment or VTM Bloodlines (Redemption was meh) but as a series nothing tops Baldur's Gate.