1. Halo 3
2. Halo 2, honestly, i think it improved in almost every aspect over Halo CE
3. Halo 5, only because of multiplayer though. The campaign was a massive letdown after Halo 4's.
The weakest one though still a really good game is ODST in my oppinion. One isuue, for me, is that those ODST's play far to much like the Master Chief or the Spartan III's ind Reach.