One of the greatest games ever.
In Halo, you are a Marine from Earth, in a giant space ship, trying to evade the Covenant, a group of aliens, determined to eliminate the human race completely. You are forced to evacuate on a strange ring shaped world, a world you soon learn to be called Halo, and it's really a weapon of mass proportions, capable of destroying all life in the universe.
The game play in this game is absolutely mind blowing. It's exciting and thrilling to the very end. There are many types of weapons with their own pro's and con's, but you can only hold two at a time. This aspect requires you to think very quickly when facing enemies. The AI, as gamespot already mentioned, is incredibly smart. Enemies will hide behind boxes, throw grenades if there happens to be many of you and your allies, and rush you with the butt of their weapons to take you out quickly. Plus, no matter how many times you play a level, things will be completely different every time.
But what sets this game from the others, is the fact that the game takes you into the story, and really connects you to it, and makes everything in the game so much more believable. Once you get to the end, it's hard to believe that you're not really on a giant fortress world, fighting a war against a human hating alien race.
(I wish I could write more about it. XD)