Halo 3 is easily the best video game ever made. The single player, thought a bit on the short side, is the most epic ride you have ever experienced. The game looks great and runs smoothly. The sountrack is also top-notch and blends perfectly with the mood of the game. The story is about a super soldier named Master Chief, who must save Earth from an alien invasion. The plot is flawless and engrossing and keeps you hungry for more even though Bungie has promised no more Halo games.
The multiplayer is also nothing short of excellence. There is no noticeable lag online. Bungie has implemented a ranking system and different playlists like team slayer, team objective, and free for all. They have also added saved films so you can view replays of every single game of halo 3 you've ever played, multiplayer or campaign.4 player co-op campaign is another added bonus over Xbox Live. Finally, the Forge mode allows you to customize your own maps, and then play them with your friends. This mode is absolutely amazing and offers unlimited replayability.
The only flaws in Halo 3 are that the campaign is a little too short, and the multiplayer maps are too big, but I'm sure the latter can be fixed through Downloadable content. All in all, Halo 3 has a hefty set of features and has established itself as the game to beat.