Here are a few of my favorites:
Star Wars: Knights of the Old Republic (1 and 2)
The storyline behind these two games is absolutely epic. Don't have to be a true fan to appreciate it.
(In Order) Halo 2, Halo 4, Halo, Halo 3 all are spectacular though.
The Halo series in general revolutionized the first person shooter game style. Another great storyline will entertaining game play and general awesomeness
Metal Gear Solid 4:
I'm not typically big into Metal Gear, but this game has got to be hands down one my favorites. It feels so real while you're playing it. As if it's really happening. Excellent game play, unique storyline, everything about it is great.