My God! Great list but the fella just trying to get into his first AR is going to read that and head straight over to the archery forum.Here's a lost I made of manufacturers for parts. Many of these companies also have their own complete rifles.
Receivers:
Aero Precision
Sons of Liberty Gun Works
BCM
LMT
Radian
LWRC
Zev
Lower Parts:
Modern Armory
Forward Control Designs
Sons Of Liber Gun Works
Sionics
BCM
Barrel:
Sons of Liberty Gun Works
Centurion
Criterion
BCM
Midwest Industries
FN
Rosco
Faxon
Ballistic Advantage
Aero Precision
Buffer and Spring:
Sons of Liberty Gun Works
BCM
Geissele
Sprinco
Handguard:
Sons of Liberty Gun Works
BCM
Midwest Industries
Geissele
Aero Precision
Bolt Carrier Group:
Sons of Liberty Gun Works
BCM
Radian
Knights Armament Company
AO Precision. AO Precision is not Aero Precision.
Expo Arms
Gas Block and Gas Tube:
Sons of Liberty Gun Works
BCM
Ballistic Advantage
Aero Precision
Geissele
Modern Armory
Sionics
Forward Control Designs
Aero Precision is typically the best receiver option, for keeping costs low.
Avoid Bear Creek Arsenal, Anderson, Springfield, newer DPMS, Omni, and any other polymer lower. Doubke check stuff that Palmetto State Armory assembles in-house.
Sons of Liberty Gun Works makes great stuff, excellent quality and quality control, and they give their stuff a lifetime warranty.