The game that made history.
More importantly though was the gameplay. This game was very different from any other first person shooter mainly because of the way every gun felt powerful. They didn't feel lame or like they were lazily put into the game without much thought. And even though you used a lot of the same weapons it just managed to never get old. No matter how many times you killed another covenant the game just always managed to feel fresh. I can't even count how many times I played "The Maw". Probably more then 20.
But of course the best part about this game that kept it in all our Xbox's for years to come was the multiplayer (co-op included). Split screen multiplayer had never been as much fun in Halo since Goldeneye for the N64. No matter how many matches you go through or how many times you get fragged or you fragg your friend it just never never never gets old.
The game also had the best single player campaign of all the Halo games. Mainly because there was always the element of surprise. Like the first time the flood was seen. But the entire campaign was also quite amazing. And every level was designed well.
If you've never played the original Halo, please, do yourself a favor and go play it. You won't regret it.