I would have to say halo 2.
For many reason. For one you have more guns. Also it allows you to peek into the covenant side of the game. Also even though its short it oddly has more detail.
The under side of it all is that the story line is almost the exact the same Halo: CE.
You fight the flood but not as Master Chief (Here by called Mc) only as the arbiter.
You stop Halo from firing. (Not as Mc) You save the day and leave a huge opening for another version of the game.
The only differences were that there were maybe 3 new levels. Bungie also admits in a press release some where that they could have done better with the game.
Graphics, although better then the first halo, sucked. Horribly sucked. Sucked to the point in which made me want to hurl my Xbox out the window.
6/8 faces were the same. Speech never differed. And who the hell thought that a plasma round would bounce off a flat side of a tank?!?
The muliplayer on the other hand was loads more detailed. I think that's the only strong point of the game was online game play. (Even though it was filled with morons who cant play
)