Depending on whether you're talking about the mass-produced guns or the small production high-end stuff.
Of the mass-produced guns, Colt or Stag Arms gets my vote, although there are various other makes like Bushmaster and DPMS that are certainly a long way from being crap. A lot of these companies use the same forgings and subcontractors, so generally all you're looking at is a few differences in the bits they make themselves and how they put them together and finish them.
The only make I really don't like much is Olympic Arms. They're a bit hit and miss, imx, if you get a good one they're okay, but I often hear about problems with them and I've seen a couple that definitely had problems with them, I saw one where they forgot to put the gas hole in the barrel!
Armalite and DPMS had problems way back when (10+ years ago), but the current stuff works.
If you get into the higher end stuff that usually ends up being a pissing contest. "Oh, you paid way too much for your super duper zapper, I got something from Jimbo's Super Guns that does the same thing for only $5,000."
E.g., Les Baer, LMT, Knight's Armament, Compass Lake Engineering, etc.
They all work, for that kind of money they should do. Whether you actually need to spend that much money depends on just how much of a gun geek you really are...