The first may have been the best, especially parts of campaign. But Halo, when you go beyond the very basic story you see just playing through campaign, actually has a fairly deep story. The books expand this a lot, but even just going through and getting the terminals deepens the story.
The multiplayer is simple, but fun, especially if you get a ton of friends together for a custom game of infection or something.
Finally (I could go on about Halo, but I won't), Halo completely revolutionized the FPS genre. While it seems less than other games in some ways, they've borrowed a lot from it. Halo CE had a good story, and many modern games really don't have that, but that started to be more common. The Big 3 of shooting, meleeing, and using grenades is simple, but requires though on how to take down, say, a group of Brutes, depending on what they're wielding, how many, etc. It's simple, but still requires some though. Of course, you can run and gun.
But my main point is, Halo, especially Combat Evolved, has really changed the FPS genre, and should be respected for that. The story, too, if you take the time to learn it, is great too. The Multiplayer, especially for serious players, may not be as fun as, say, COD4. I'd say that COD's multiplayer is better, for serious gaming. But the campaign's story wasn't as good, and there are no more relaxed gametypes, like Infection, VIP, Juggernaut, Oddball, etc.
Long post. In short, I really respect what Bungie has done in the Halo Games.
[url=http://www.youtube.com/watch?v=Yff3jH8NECs]"Touch my Awesome Button."[/url]