ODST and Reach are my two favorite Halo campaigns probably.
I really enjoy the story in both, to me the Master Chief as a character is practically a superhero and the ODST squad was more grounded as people and the Spartans in Reach were more diverse and flashed out than the Chief.
And from a game play perspective I really enjoy the lack of Flood levels. In 1-3 I despise the levels where you fight The Flood, especially in Halo 3.
I loved ODST. That scene in the beginning where you get dropped in the pod caught me into the feeling of being an ODST right away. Then sneaking around the dark streets with friends to find clues.
It was different from Halo 3 in an interesting way and I liked it. I wouldn't want a sequel or anything, as I think that ODST was enough of that.
Reach felt... meh. It was fun at first, and quite interesting. But honestly, the only map that I enjoy replaying several times is the Long Night of Solace, because of the space spart.
The missions generally feels boring and there arent many memorable moments in the game at all. None of the Noble teams members feels like they have personality beyond "I'm badass and I'm going to die a noble death".
There were some moments that I liked, but all in all, I never felt like I was there on Reach.
In the level tip of the spear, you really should have joined an armada of vehicles while rasing through the barren plains while trying to reach your goal, mowing down coventant forces and vehicles while seeing the tower get closer and closer... not walk half the way there and getting distracted all the time.
Reach was an experience, but it wasnt what I wanted it to be. The multiplayer was okay, but I didn't like the experience/unlock system. Felt like I was playing to unlock ranks instead of having fun.
I'm hoping that Halo 4 will be better missionwise, and it probably will be, because there is a story where you don't know how it will end before you even start playing.
I really enjoyed everything ODST did. The campaign was somewhat unique. the Terminal sub-story was wicked awesome. The firefight mechanics, and succeeding with my bro and his friends was way more challenging than Reach's firefight, and the multiplayer disc and forging on ALL those maps consumed most of my days.
Unearthed was mainly just cool to look at. It was a totally weird map though and it only utilized some 25% of its entire size for the most part. I would've much rather played in the multi-leveled canyon and had the processing plant as a background. WTF didn't they do that?
My hypothesis as to why ODST had some things that were better is that it seems like Bungie tried to go out with too big a bang with Reach. They literally included everything you could possibly imagine, and that's great, but it also gives us too much flexibility and not enough challenge. Endure is possibly the best achievement I've ever gotten; it was incredibly difficult yet satisfyingly fun at the same time. The only thing like that in Reach was Monument to All Your Sins, which wasn't even as difficult because you didn't have to depend on three other people. On a somewhat related note in my rant, another thing that broke multiplayer is the fact that they had Halo 1 and 2 gamespaces in a game that has far different mechanics, thus breaking these incredible maps. The prime example of this is the fact that you can practically kill someone a mile away (or so it seems) with the DMR, and there are clear sightlines between the two bases on Hemorrhage, thus making things suck, unlike in Halos 1 and 2 (it is also next to impossible to kill vehicles without another vehicle, seeing as the power weapon is a POS).
Long story short, Bungie over-innovated and did too much without realizing the repercussions. ODST just felt like it had more polish
I completely agree with you, In fact, I'm of the opinion that if Bungie had been given and extra year, or even a mere six months, they could've fully realized Reach and made it one of the best games of all time.