lol I know this is unpopular on these forums, but I've been playing Halo 3. I've played through Halo 1 and 2, and finished 3 yesterday. I really like this series. I'm looking forward to Halo 4 now!
The first game was just fantastic. I never had an original Xbox, so I never got what everyone was raving about with Halo. It's solid from start to finish, and just feels great to play. (By the way, I'm only talking about campaigns here. I don't care about online multiplayer so much, though that wouldn't be possible with the first and second games anyway at this point.) The final stage is really satisfying as well.
Halo 2's story set up 3 very well, but as a standalone game just didn't impress me nearly as much as the first game. Probably because it kept switching characters as the stages progressed. I liked both characters, but neither of the stories, though they eventually merged in the end, felt complete like in the first game.
3 balanced the weapons perfectly, and was storywise a very satisfying conclusion to the trilogy. I enjoyed it much more than 2.
I remember when Halo 2 was released, and everyone and their great grandparents wanted to know what I thought of the damn "Flood level". That game just did not live up to the insane amount of hype it received. I do think Halo 3 did though. I was very impressed. Hopefully, 4 will be equally satisfying.