As for "what"...it depends on what you want to do with the rifle. Flash hider, muzzle brake, suppressor adapter...gotta have the right tool for the right job!
I recently built up a 16" mid-length gas AR for three-gun fun, and general use. Basic parts were:
1994 vintage Olympic Arms lower
DSA flat-top upper
RRA two-stage match trigger
Hogue grip
Rifle length ACE skeleton stock
White Oak 16” ss barrel w/ mid-length gas
YHM two-piece gas block
YHM diamond mid-length Forearm w/ Magpul rail covers
and - a PWS FC556 pinned/welded compensator (to stay legit here in CT). I like the Primary Weapons System comp/flash hider.
Burris PEPR w/ Burris compact 3x9 and a Burris FastFire II piggybacked for close in work.
My AR has an AAC blackout as well, in preparations for the day I finally get around to ordering a suppressor. Damn thing works pretty well as a flash hider as well, and lives up to its name.
I had a Bennie Cooley compensator on my target AR but I traded it. Two of them have A2s. My AR10s have a Phantom and the other is just plain. I haven't shot the Bennie Cooley compensator to say whether or not it makes a huge difference of not, but it is noticeable no question. Much louder too. The Phantom is very nice on the 308, it takes the recoil down to something very manageable.
Its not on a AR but I love the muzzle break on my MSAR Styer clone, that rifle has less recoil than any AR I own! Maybe its just the wa the gun is designed??