Titan Quest is more or less Diablo II. An RPG, it's not.
On top of the list should be the following:
Planescape: Torment
Baldur's Gate II (it's better than BG1 - but fuggedabout Dark Alliance)
Fallout II (Fallout 1 is not as good, but if you want to play it, play it first)
The fact that these are all Black Isle games is not a coincidence.
If you're willing to broaden your definition of RPG and figure out a Dreamcast emulator, Shenmue (and Shenmue 2) might fit your definition. I've never played either, but these games have truly amazing reputations.
On a more negative note, I played a good chunk of Icewind Dale II, and hated it. A Baldur's Gate style RPG is dependent totally on story and character, without which the game becomes a mediocre squad-based combat sim.
Also, The Witcher is not entirely an RPG either - Yahtzee didn't much care for the bit of it he played, so I recommend trying before you buy.