Baldur's Gate 2: Shadows of Amn
I honestly have trouble thinking of a single thing I dislike about this game. The story is suitably dramatic and well told, the villains memorable, the supporting cast brilliantly brought to life, the combat engaging, the visuals stylish, and even the music is fantastic.
It is, hands down, the greatest RPG ever made in my oh-so-humble opinion, and a game I think anyone who even remotely enjoys the rpg genre should play. It puts everything else to shame.
Ten years running and still king of the genre!