Just played through all of them except Reach and I'm currently on my second playthrough of Guardians. I'd say Halo 2 AE is overall my favorite, story was delivered nicely, I liked Arbiter's character development and gameplay was quite catchy.
Halo 5 has the most exciting and dynamic gameplay though, sprinting and new abilities make Spartans feel like they mean some serious business. Halo 4 was also pleasantly eventful.








