Anyone who voted for pong is either doing it because of the fact pong is the first major video game success (which is stupid), or to spite GTA IV (which is also stupid). I say this because there is no sane person born after 1960 that would honestly rather play a game where you hit a tiny white dot back and forth between two thin white lines for hours over a game where you get to drive over a realistic New York City landscape in any sort of car you could imagine, fire guns at whoever gets in your path, fly over the skyscrapers in helicopters... fuck man, you can do ANYTHING in GTA IV. Even the minigames in GTA IV (pool, bowling, darts, QWEBED) are more entertaining than Pong.
Not to mention that Niko Belic is one of the most interesting video game characters of all time. For the first time GTA is given a protaganist that would much rather be in a legitimate line of work, but unfortunately for him doing errands for gang lords seems to be the only work he can get. He deeply cares about his cousin and his friends, and sacrifices his life on a regular basis to try to help them. For the first time ever, the main character of a Grand Theft Auto game is sympathetic.
And above all else, the missions in this game are awesome. You have your heist mission (which blows the previous heist missions out of the water), a shoot out at a museum, chasing down a mob boss to an abandoned casino, and countless other epic missions that are some of the best missions in the entire series, if not entire genre.
I'm voting for the obvious choice here, GTA 4.