Halo 1 (I got it for the PC) I thought the campaign was good but lacked memorable levels, maybe I'm biased because I bought it in like 2008. The multiplayer, obviously, wasn't too good either.
Halo 2 I thought was an improvement in every way, the campaign had memorable levels (The Breaking Benjamin Room, no gravity section, fighting the Brutes as Arbiter) albeit with some extremely terrible levels as well. The addition of the Arbiter gave it much more story depth and made me annoyed that you couldn't play as him in Halo 3 (I bought Halo 2 about the same time as Halo 1 by the way). Finally the multiplayer was a HUGE improvement and the maps were great.
I bought Halo 3 first out of the series when I first got my 360 and the campaign alone was good enough to keep me playing for the year before I got Live but I really had no idea what was going on story-wise. When I did get Live it took me a while to get the hang of it but ended up playing it the most out of any of my games until the release of the Mythic map pack after which I stopped playing since most of the games weren't allowed since I didn't have it, which annoyed me quite a bit to say the least.
Never played ODST.
Got Reach a couple weeks ago and I think that the campaign suffered the same fate as Halo 1, lack of memorable levels. I mean yeah the space combat was pretty good but the 5 minute tutorial you had to play through everytime you wanted to play it kept it from something you could play on a whim. Not to say the campaign wasn't fun, just that nothing stands out when everything's shining. The multiplayer was again, an improvement in every way as far as I'm concerned, Forge has made some incredible custom maps even if Forge World was over-hyped and Firefight will give me hours of fun long after the campaign's gone stale (Gruntpocalypse FTW)
I've written way more than I intended for this :S