I don't know about best computer rpg, but here are some I liked:
Septera Core if you want a traditional party based RPG. It's old now so I don't know if you could actually find it easily.
Anachronox which is not a traditional rpg at all but is pretty fun, althought I haven't finished it. I rarely finish games these days.
KOTOR One of my favorite computer rpg, I played it trough at least 4 times, it has to be a record for me.
KOTOR2 same as above.
Divine Divinity If you're looking for an action rpg.
EDIT: Oh yeah, Arcanum was nice, it's a buggy fantasy version of Fallout, but it take some time getting used to it. Whe I first bought and tried it I hated it and shelved it, a year later I got it out and actually enjoyed it this time.
[ 09-08-2006, 01:43 AM: Message edited by: Luvian ]