This spot used to belong to Portal until the second game came along. This will probably be the most "mainstream" game on the list. Building upon every aspect of the first game, Portal 2 managed to make the perfect game even more perfect. Like yesterday's pick, I'd have to say Portal 2 is easily the best game of it's genre.
Portal 2 is, as many of you are probably aware, a first-person puzzle game designed much like a first-person shooter, except instead of bullets, the gun shoots portals. On top of this ingenious game concept, the game is only added to by extremely humourous dialogue, clever puzzles, excellent graphics, gameplay, and sound, and a kickass credits theme. I will say that Portal 1 probably had the better credits theme of the two, but hey, it's hard to beat Still Alive.