I've heard good things about Rock River's "accuracy" models:
http://www.rockriverarms.com/item-detail.cfm?ID=AR0500X&storeid=1&image=uva407.gif&CFID=35336013&CFTOKEN=84141066
They have 1 in 8" twist, stainless bull barrels with the Wylde chambering so you can safely shoot either .223 or 5.56. I'm still on the fence about whether to do an M4-style carbine or a looonnng-barreled varmint gun, but these uppers are on my short list.
http://www.rockriverarms.com/item-detail.cfm?ID=AR0500X&storeid=1&image=uva407.gif&CFID=35336013&CFTOKEN=84141066
They have 1 in 8" twist, stainless bull barrels with the Wylde chambering so you can safely shoot either .223 or 5.56. I'm still on the fence about whether to do an M4-style carbine or a looonnng-barreled varmint gun, but these uppers are on my short list.