Some people have a different measurement of "well built" than others do. I think that's what makes it such an interesting topic.
No matter who you are, well built starts with a good barrel. I prefer light weight pencil barrels while others like heavier barrels. Neither is wrong as long as they serve the users purpose and are gassed right. I have had good service with Sioncs and Colt for pencil barrels. They are not sub MOA but are chrome lined and will scare 1.5 MOA usually with the 4 samples I have on hand. All have been mildly gassed with the edge going to Sionics but it's a mid-length. My Colt carbine gassed barrels are smoother shooting than mid-length Faxon, PSA and the LaRue barrels I've tried. The LaRue barrel is pretty accurate and still has a place in my safe.
Gas blocks need to be steel not aluminum, pinned or at least a divot drilled for the set screws. Adjustable blocks can cure an over gassed barrel but I'd rather start right if possible. If you're using a suppressor, they would make great sense.
Muzzle devices, I have seen one occasion where an A1 flash hider caused a match grade barrel to shoot patterns. Barely torqued or heavily torqued. Stuck a Vortex on there at the range and the barrel settled right down and shot great. I've played with a few brakes and really favor the Vortex but it's not the only solution.
Next you need a good BCG. I like Colt, BCM, LaRue and Sionics but I suspect they are made my other companies and sold by several. The difference is probably going to be QC specs but if you get a good one, it's a good one.
Uppers and lowers as long as they are in spec, are not going to matter much. I loctite my barrels into the upper and don't worry about truing them, maybe I'm just not that grade of shooter.
Triggers are subjective but I prefer Geissele 2 stage triggers for the most part though I have a couple of their single stages that I like on purpose built guns.
Rails, I have Troy, BCM, Midwest Ind and even a MagPul SL hand guard. They have all served their purpose well but If I had to pick one it would be the Troy Alpha rail (it's really a tube not a 4 way rail most think of). It's solid and robust and I never worry about it. My BCM is the magnesium/aluminum hybrid that works fine but would bend easily, it's greatest attribute is it's light.
Grips, I prefer K2+'s and stocks I like LMT SOPMOD's over all others.
The only factory built gun I have is a Colt and I've made changes to it. BCM, DD, Sionics and probably a few others would be a good choice too but I prefer to build with good parts to suit my tastes.
The best advice I can give is to start with a great barrel for your uses, everything else is subjective.
I've used, owned, worked on other guns like PSA and for the most part they will serve one well but I've yet to see one shoot out of it's price range.
