Morrowind, I tried playing that on my Xbox (got a modded copy off the net of the "GOTY" version with extras and stuff). I played it for a few hours, but was turned off by the dated graphics. I bet if I would've played it years ago when it was hot shit I bet I would've loved it! Or even if I played to play the PC version today modded with these modded high-res textures and stuff that i've briefly heard about.
Oblivion, seen it, looks fucking beautiful. I want to play it so bad. Alas, I have neither a power-gaming PC nor an Xbox 360 :(