Halo 2 trumps all other Halo releases, by far IMHO. It was the most pure multiplayer fun I had in a shooter, ever. All the subsequent ones just added too much stuff without adding much value. If they can bring that back and up it for the Xbox One then I'm all in!!
I think 3 was definitely the last core-Halo MP that I enjoyed. The drops added just enough that it didn't feel overwhelming but added lots of new strategy. I will never forget seeing the bubble shield for the first time in the Superbowl ad
Halo 2 had the "next gen" feeling when you played it online for the first time. Was so far and away from everything else MP-related on consoles. Nostalgia plays a big part here in looking back, but even the community when it launched was amazing. It turned to **** a few months later unfortunately
Reach is good too but for lots of different reasons and even with its issues 4 can be fun, I just ignore the loadouts and play it "classic" or pretend it's Reach even if I'm at a disadvantage.