The old school of thought was:
A - armalite
B - Bushy
C - Colt
D - DPMS
R - RRA

Now days, technology has changed so much that a guy can take an 80% lower and mill it into a legit 100% stripped lower. I'd review/research each vendor from this site:
http://www.ar15.com/lite/topic.html?b=3&f=12&t=318113

I prefer the following now:
DSarms - cost per performance
Mega - 1st pick if it's available.
RRA - 2nd pick, since the majority of my uppers are RRA.