Halo 1 you're fighting the covenant and trying to discover what they want with the halo's Once you find the flood and discover what they are you're objective is the blow it up.
Halo 2 It sets the backdrop for how humans are going to win the war. The covenant aren't as unified as humans think they are.
Halo 3 It ends the flood and leads to the defeat of the covenant.
I won't touch on ODST. I have it for firefight online
Reach. You're defending reach then at some point you realize reach is gone so it's get cortana to the autumn.
Halo 4. Brings us the forrunners and shows us that humans just may not be the good guys.
Why make Reach a fill in game. They could have expanded master chiefs character or shown us just how awesome keyes is. Had I not read the books I'd have felt nothing when we find him dead by the hands of the flood.
Reach has a thousand stores that could have been told yet we were stuck with a half assed one.