Best .223 Semi Auto for Varmits and plinking?

I'm looking for a .223 AR-15 for prairie dog hunts and plinking. I've looked at rifles by Bushmaster, Rock River, Armalite, DPMS and Colt. Figuring a 18 or 20 in barrel would be nice. Not sure which features are most useful. Budget is around $2500 including optics and whatnot Any advice?