I don't like making lists, since my preferences tend to waver depending on whichever I'm enjoying more at the moment. But here's five of em I consider my favourites in no particular order...
Except for Baldur's Gate 2: Shadows of Amn, that games definitely my #1. Play it if you haven't, you're missing out.
Other than that:
-Vampire: The Masquerade - Bloodlines
-Knights of the Old Republic 1/2 (I'm torn as to which I prefer!)
-Neverwinter Nights 2: Mask of the Betrayer
-Elder Scrolls 3: Morrowind
-Planescape Torment
Guess that's technically 6 picks if you count BG2, and 7 if you count both Kotor games. But whatever! Plus if I were to take user made content and mods into account, I'm also a big fan of the first Neverwinter Nights as well although out of the box I found it to be more than a little lacking which is why I only count it among the greats if mods are also taken into consideration.