I probably had the most fun with Mario Kart Wii, but I went ahead and voted for Metal Gear Solid 4.
I'm not a particularly big fan of the MGS series: Really enjoyed the first game, did not like the second game at all, and thought the third game was very overrated. But fortunately, the control of the character and camera became, well, good, at the same time that the game itself successfully embraced the series' weirdness and gave us a crazy story with cattle-sounding bipedal metal gears, the Beauty and the Beast corps, and managing to make Raiden cool. Just make sure to bring the popcorn. Considering it's still restricted to the PS3, this game is just begging for a proper remaster.