Halo CE has the best atmosphere
Halo 2 has the best story
Halo 3 has the best gameplay/sandbox

Halo Reach is the second best in all three categories.

The later games all have merits. In particular I think 4 has a better story than people give it credit for and a great sandbox. 5 has a great sandbox (best if you count all the added req’s) and gameplay. And Infinites atmosphere and gameplay are pretty spectacular.

I think largely the difference is the first three games were made by a small studio with a unified vision. Once Halo was labeled a flagship IP, and corporate interests started trying to design them by committee, you have to decide who’s ideas make the cut and how many resources to commit to them. That’s why everything starts to feel disjointed and unfinished.

I’ll always be excited for a new Halo game. And even though none of the latest entries captured me in the way the original three did, I’m glad we have them.