6) Mass Effect 2
The original is a great game, but the sequel is where the series was perfected. The action is far improved, the character work is more interesting, the scale is much larger and, gosh, My Favourite Store On The Citidel, right?
Mass Effect shows you the political ramifications of alien life foremost intending to look after their own interests. It's a world where people from distant planets aren't painted as evil, more that they're self serving. Your role as Commander Shepard is to find a way to make situations beneficial for everyone, and if you can't, you punch reporters and throw people out of windows.