Geometry Wars is fucking awesome. I still remember playing Project Gotham Racing 3 on my 360 as a teen and discovering the Geometry Wars arcade cabinet in your in-game house that just lets you play the whole game.
For me, the most recently, probably Dishonored. Id played it through before, and really enjoyed it, but never thought about replaying it until I saw it in my library again. So I replayed it, and goddamn it, that's a great game still.