[Sever]

I'm giving my vote to BG2 on a technicality. It's clocked up more hours of my free time than any other game (rpg or not) that i've ever played. Great story and character development but, most of all, exceptional combat and magic mechanics. It's like a game of chess every time you get into a scrap! Basing a rpg on a tried and tested (and revised and improved) rule set is a guaranteed winner.

A close second, in terms of merit, not hours (yet!), goes to Morrowind. No other game gives you the sense of freedom to explore and develop your character the way you want to. It puts the role playing into rpgs. Unfortunately though, Morrowind falls short of BG2 in the combat department. Aah! But it's not all about combat!