>KotOR
>Legacy of Kain
>Planescape
Ron Whitaker, you are a gentleman and a scholar.
Knights of the Old Republic, while in hindsight is the first step towards "Standard BioWare RPG", was still the first game in which a plot twist made me legitimately shocked (although I played the game without prerendered cutscenes back then and missed half the foreshadowing, I admit). Even if I have beaten it a dozen times and saw each damn quest in the game, I still sometimes fire it up and go into the world of lightsabers and swoop bikes.
Planescape: Torment... I sadly haven't beaten it yet, because I kept dying at the final location - a really bad set of circumstances led to me being down to a single life and the Fortress being overrun with the shadow beasts. Still, most of the story was simply great and I will play the game again and beat it, to see how it plays out.
Now for the big one:
Legacy of Kain is... probably my favorite video game series of all time. It has it all - vampires (not sissy ones, but badass monsters), fantasy, time travel, flowery shakesperean dialog and a gripping story that has more twists and turns, than a Tim Burton pic. Kain and Raziel are two of the greatest anti-heroes of all time, both sympathetic, but not too much, as they carve up legions of enemies each with their own version of the Reaver sword. The series keeps playing with audience's perspective, challenging their view of past events and characters, only dropping the ball once for Blood Omen 2, but picking it back up and going for touchdown in Defiance. And they are beautifully voice acted to boot, with Simon Templeman (Loghain Mac-Tyr from Dragon Age, Dr. Gavin Archer from Mass Effect) providing Kain's voice for all five games and Michael Bell as Raziel. The great voice acting doesn't end with two leads though, as we have late great Tony Jay as The Elder God (games 2, 3 and 5) and Mortanius (1), Richard Doyle (all save for BO2) as Moebius and Anna Gunn (yes, of the Breaking Bad fame) as Ariel. The series is, simply put, a masterpiece and I'm in the middle of a LoK marathon right now, playing in parallel both Blood Omen and Blood Omen 2. To the PlayStation owners I suggest picking Blood Omen and Soul Reaver on PSN and the rest of the games on either Steam or GOG - you won't be sorry.