Medieval 2 total war... but a lot of people seem to like the old Rome total war more... if you can afford them get them both but Medieval was best in my book so I have to recommend it even if others don't seem to agree.
Empire earth was one of the best RTS's in my book, as well as being quite an unusual adornment for any gamers shelf.
If you like action-RPG's I'd recommend Fable 1... and possibly Fable 2, though I'd only recommend that if you're a proud possessor of XBOX live or a PC (if it's out for PC.)
I'd second the Diablo recommendation. I can't comment on 2, but I enjoyed the first. (though I couldn't be bothered with nightmare, let alone hell.)