For plinking, I'd use any ammo that's cheap out there. PMC, Wolf (both of my 6920 eat steel cased ammo like kids eat candy, but my 6520 hates it - don't know about the 6721 never tried steel case in it), UMC, AE.223, Black Hills blue box, Ultramax etc. I find Black Hills, PMC and AE.223 to be the most reliable with the UMC being somewhat underpowered and cause short stroking - not a bad thing for plinking - give you the chance to deal with a malfunction. Ultramax short stroke too often in all of my ARs - except for the overgassed Olympic Arms, but I still use that stuff in my 6920s when shooting from the bench building groups.
It is my opinion and my opinion only: I wouldn't feed an AR with carbine-length gas system fully gassed ammunition all the time just to be kind to the gun's internals; that is why I use lower pressure stuff such as PMC, UMC and Ultramax for plinking and running drills. Like I said, UMC short stroke once in a while, but for plinking / practice so what?
For self defense or zombie fo time, XM193 (or any of the M193 type) at the minimum. If you had the $$$ read the ammo oracle for serious use ammo such as 75 grain TAP. I am broke, so my go to ammo is XM193 or Q3131 (whatever that thing is called the Winchester 5.56 white box). For self defense ammo, my first priority is reliability - I want to make sure that the ammo will cycle the gun 100% - frangibility comes second, after all shot placement comes first.