Oblivion! If you get it on the PC then you can download some mods which extend the game beyond vanilla. I'd recommend OOO which makes it ball achingly harder and a lot slower, Lost Spires which is an awesome mod that adds an Archaeology guild to the game which you can join, and there are some others which improve the combat system dramatically but, to be fair, I had trouble getting them to work correctly and gave up on the combat stuff. The rest of it was awesome though and I'm still playing through. 30+ hours so far and I'm only level 9. :/