I'm gonna go with The Dig as my nomination. Excellent point-and-click adventure game with an interesting and original plot, and decent puzzles to boot.

Baldur's Gate for me was overrated. If it was half as long then it wouldn't be, but the game just drags in my opinion. I agree with the Planescape and Soul Reaver nominations though - excellent games that for some strange reason have only become as popular as they deserve in more recent times. Funny how some things go.