ODST at the moment. I need to go back and play legendary.
I'm constantly going back to play bits of CE, but have really been in the mood for its multiplayer lately. Unfortunately the MCC version's multiplayer is not particularly playable so I'm usually playing Custom Edition on PC.
Halo 3. Replaying "The Covenant" on is the best mission to come back to in that game. Also it has a better ranking system and armor unlock system than the other Halos (in my opinion).
I still play Reach, the only bad of it.. It's that is too competitive, I don't mean this is bad an I hate it, but people with I'v played car more about wining that enjoy it he game. Stills being and awesome game.
One of my favourite parts of the original series were the parts in halo 2 when that amazing electric guitar remix of the theme (mjolnir mix) started like when you take down the scarab for example.
I still play all of them when I have time. Though i play wars the most, I like strategy and a challenge and wars always presents one. Halo 3 odst i would say is a close second, mostly firefight.
Really don't play my 360 much as all the main Halo games are on Xbox One, and all 360 games I like to play are now backwards compatible. However, I will be revisiting ODST on the 360 at some point to finish up the firefight achievements.