First off, this is merely my opinion, take it as you will.
Halo, original, granted, a good game. Though the idea has been done before, they brought it forth well, had good graphics for its time, and was enjoyable when you had some free time. But it was not revolutionary to me. The storyline, kill the alien invaders, to be general, isn't exactly new, you know.
Halo 2, a shinier version of the original, with more weapons, watery plot line.
Halo 3 Beta, a shinier version of Halo 2, with more weapons.
My thoughts? Liked by too many, and cast a shadow on the Xbox games that most people will now never play.