I think the obvious ones have been mentioned, I would like to throw in A Bard's Tale, which is a musical comedy RPG
I'd also at least add the original Dungeon Siege to that list.
Originally Posted by Avery
If we're going to include customisability of weapons and armour as the base requirement for an RPG, then Assassins Creed 1 and 2 are my picks for that category.
Last but not least, Final Fantasy VII and VIII did exist on the PC, so are also on my list as an essential PC RPGs