There are quite a few primary weapons in the game to choose from. My personal one is the Assault Rifle. I have worked through all the Halo’s (except Halo 2 :c) with that weapon. It’s a good favorite to me. Now back to the question: what is your favorite primary weapon?
Probably the DMR.
But like you, the AR is wonderful. Very nostalgic and fun.
I like to use “Firepower” and use those two. 
DMR for everything except for Abandon. I use the BR when that map comes up.
I’m a bit down the middle with the AR and the DMR. I LOVE using the AR because it has gotten MUCH more useful in multiplayer than the past games. In small one-on-one battles, the AR usually comes through for me.
But now, since most of the maps are huge, it would seem more appropriate to use the DMR. It’s pretty difficult getting all the way up to an enemy where the AR can be useful without getting attacked by dozens of DMR and BR bullets.
So I’m kinda in between those two.
DMR 4 Shots
Carbine 8 Shots
DMR Primary
Carbine Secondary
EDIT: nevermind
The battle rifle. I like it over the DMR because of the three-shot burst.
DMR. It kills the quickest and is best at long range.