what i buy some label it "in the middle", but the parts just keep working! i won't list what i'm NOT a fan of, i'll only list what i buy or had trouble with. i've been building ar's (both of 15's and 10's) for 10 years, so some manufacturers escape my mind.
aero precision for any ar15 or 10 has been very good
faxon is hit and miss
cmc is a good low dollar trigger brand (i have several that hasn't failed)
timney, elfman, trigger-tech for cassette type (don't buy much individual types) triggers
barrels........McGowen, x-cal, aero precision, criterion, bsf, proof research and a few other i don't recall (i have several brands)
gas blocks..........slr, odin, aero precision and 1 other i can't recall at this time.
bcg's are all on the upper side of high dollar although many have been bought on sale.
mags......the only mags i've had any trouble with the promag (mainly with pistols, but a few in ar's)
receivers (upper and lower) from $29-350 have all been good including poly type until last week.....1 ear on the pivot pin (lower) broke. it lasted 10 years.
not sure if you're here to gather info or actually stay as many new peeps have been trolling!