Armalite AR10A. But these can only be had with s complete rifle at the time being. Very solid forgings, superb machining, and rock solid QC.
For around 900$ you can get a complete DEF10 rifle, sell off the barrel, and your right around 650$ for s complete receiver set minus the barrel.
Already has a great buffer system, unless you want a rifle buffer system, swap out the GI stock and GI trigger, and you're set. Depending on your barrel and handguard choice, you'll be under 1500$ for a complete build. Then you gotta pick sights or optics, or both.
If you've got a 2000$ budget, you can make a very nice Armalite. Granted, the receivers may not be Gucci looking stuff like Mega and Rainier, but they are very stout, very rigid, and very well made. If you want fancy-schmancy stuff, MaTen or Rainier.