I have yet to play the Mass Effect series, but I agree with everyone else when they say Fallout 3. I don't think I've ever invested so much time into a modern day video game as I did Fallout 3. I really enjoyed New Vegas as well, but never completed it due to all the freezing. If you can get past the bugs, glitches and freezing you should be good.
I accumulate pieces of cardboard that happen to feature Luke Kuechly on them...