gyokzoli: Probably either Planescape: Torment or Icewind Dale.
Both Icewind Dale and PST will be the same every time you replay them. Sure, you can change your party and completely overhaul your tactics in IWD, and make different decisions and get a comletely different character in PST, but the progress will always be generally the same.
Psychonauts is a game that most players will only want to finish once, there's no freedom to speak of and difficulty curve is not exactly smooth.
Nightfall87: Alpha Centauri, followed by Colonization. In case you're fan of 4X games. :)
... so, this, or another 4X game. If that won't work for you, you might consider getting Heroes of Might and Magic 3: finishing all campaigns and custom maps will take you ages,