Hands down the best 9mm bcg and buffer setup on the market - JP Enterprises EnhancedBolt 9mm AR Bolt Carrier Group (BCG) Kit - JP Standard Silent Capture Spring

I decided to go with this setup because I wanted my AR9 to function flawlessly. I went with polished stainless steel BCG which is super smooth and well engineered. The only drawback for me was the price, but if you want the best, its going to cost you.