I'll add another 2 to my list:

Assassins Creed 2
I have the first one and I loved the whole game concept. On the first few assassinations I planned out elaborate hit and escapes, explored the city and was having a great time. Then I discovered you could just walk up to your mark, kill them, take on 20 guards at once, and all the citys were essentially re-skinned versions of themselves.
The review of Assasins creed 2 I read said it made AC1 look like a tech demo, which sounds like exactly what I want but I also have learned to not trust language like that. Do the missions in AC2 make great use of the game engine, or is more typical sandbox make your own fun?

Fallout 3 / New Vegas
I'm not a fan of WRPGs (Elder Scrolls, KOTORm etc...), but I have a friend recommending this one to me. It's also the same friend who recommended Borderlands to me despite the fact I don't like FPS game, and I absolutely loved that. If I were to get one which would your recommend?