Personally, my favorite ar grip - Ergo Grip AR15/M16 Ambidextrous Tactical Deluxe Rifle Grips

Purchased this grip for my newest AR - a Rock River Arms .308. Did so immediately after ordering the rifle. Just as I've done with every other AR I own. I simply find this Ergo-Grip preferable to any other grip on the market. I find it to be more comfortable. It fills my hand better than the standard A2 grip. And gives more purchase than any aftermarket grip I'm aware of.
So, a new Ergo-Grip is the very first thing I order for any new AR. You may feel differently. Every hand is different. It's really a matter of personal preference. But, so long as Ergo-Grip makes them, I'll continue to buy them. Their fit & finish is outstanding. Customer Service goes above & beyond. And I've yet to have an issue with the performance or durability of an Ergo-Grip.