ODST is my personal favorite campaign. The music, atmosphere, missions... I love it. As far as the other campaigns go, here's how I rank them:
1. Halo 3: ODST
2. Halo: CE
3. Reach
4. Halo 3
5. Halo 2
6: Halo 4
Multiplayer-wise, all 3 games in the trilogy are great, with Halo 2 being the standout IMO. Reach... is pretty bad. It has the worst maps in the series and it added armor abilities that slowed down the game. The movement felt bad too. As for Halo 4, I loved it at first but as I played it more and more, it's flaws became more apparent. Halo 4 improved on the mobility and map design (slightly) but it really did everything else worse. Ordinance was a terrible idea and I hated the whole create a class system and the maps were only a small improvement over Reach. It doesn't feel like Halo at all.
I hope 343 learns from their mistakes for Halo 5.