AR-15's , I haven't been to one NES shoot (And I've been to many) where I haven't seen somebody having an issue with an AR.
AR's are like Bitchy Women, High maintenance, No matter how much You care for them they will still come back and bite you when you least expect it.
I'm sure I'll draw fire for this post but I'm not worried, your AR will jam while I'm on your flank.
My M&P15 has never jammed even once in about 1000 rounds. And stay away from my flank!