I played Arcanum for about twenty minutes once...it looks kinda neat.
Greatest computer RPG ever remains Septera Core: Legacy of the Creator.
People have already said it but it needs saying again, any RPG fan must play Fallout 1 and 2. Forget the original Baldur's Gate though and just go straight on to Shadows of Amn, it's superior in every way. Then there's Planescape:Torment, they don't make them like they used to.
I quite enjoyed Divine Divinty too.Oh and Knights of the Old Republic.