I liked Halo 1. It was fun on the Xbox, but since I'm not a console gamer, I picked up the PC version about a year ago. The cut-and-paste level design is a far cry from, well... Far Cry, as well as other great PC shooters like Half-Life. However, it was quite fun: there was a lot to do, usually, the weapon balance was good, and it didn't do that irritating thing where you can carry 12 guns and all their ammo in your pocket. The melee attack was a nice addition that has been copied in quite a few games since, and although some people didn't like the recharging shields, I didn't mind. I thought it fit the game well. It's an action game that does not pretend to be realistic. The story is not great, but it's passable, and at least it's there, which already puts it above many other shooters. The multiplayer was fun, and actually resulted in some fairly tactical on-on-one battles, which is a very rare feat. Unfortunately, it's really only fun when you're playing with friends, because the online community are all incredible assholes, and no matter which gametype the map is set to, all they want to do is play free-for-all Slayer. Also, half the servers were always set to "Rocket Launchers Only" which completely removed all the tactics, and therefore most of the fun, from multiplayer gaming.
Halo 2, I only played at a friend's house, so I did most of the campaign on co-op. Some of the battles were quite fun, but overall the game was not as tight. The new weapons weren't quite as balanced, and the story made little sense. Environments were pretty, and animations were improved, but I kind of liked Halo's too-fast animations. I guess what I'm saying is Halo 2 had more style, but less charm, and ultimately, less substance.
I haven't played Halo 3, but it's fun, from what I've heard, apart from being really short. I think Bungie probably corrected their mistakes from Halo 2, though I doubt they reached the same heights as Halo 1. I wonder where the Marathon designers went? I heard that a lot of them jumped ship when Bungie was bought by Microsoft...