As it seems with many others in this thread, Planescape: Torment is still the gold standard against which I measure the many games I have played since. None quite measure up, but some come close. The obvious ones are the Fallout series, the Baldur's Gate series, Deus Ex, Might and Magic 6+7, and the System Shock series. However if you want early 90's games to try out, I still remember playing some great RPGs on my old 486. These include:
Darklands
Wizardry Series
Ultima Series (Especially the Underworld games - try these if you see what inspired Deus Ex, System Shock, later Elder Scroll games)
Betrayal at Krondor
Jagged Alliance Series (as Tactical RPGs)
Proud to say that Final Fantasy 3 (FF6 in Japan), was the only JRPG to keep my interest long enough for me to get at least 2/3 of the way through.