My favorite is Eagle Arms(Armalite), Sabre is nice, RRA (not for uppers), Bushmaster has good lowers.
Most lowers are only finished by the manufacturer stamped on the side, the forgings all come from a limited number of places. So like the other guy said, it all depends on whether you want it to look nice, shoot nice, or both.
stay away from DPMS, they are a descent company that makes a descent rifle, but thats never good enuf for me! there lowers are a few $ cheaper but none of my mags dropped free from mine, not worth saving 20 dollars IMHO!
Ameetec, Superior, Double star, essential....other descent lowers at good prices.
Oh yea Denny at Global Tactical has great looking lowers, but I think many places are out of stock right now.