Most manufacturer stripped uppers are fine... ones i would buy would be (BCM, Stag, Spikes tactical, CMMG, DD , LMT, etc..) I have a CMMG lower married nicely to a stag upper. Only thing u have to look out for is really an aesthetic thing with the actual finish. CMMG recievers have an extra coat on them that gives off a slight shine which i find to look good. Most manufactures make them flat finish like my stag upper.