LRB would be the top pick if you can afford it.
While LRB's are good, I opted for a Fulton as I've been able to get them for under $400 when they've been running a sale and I used the $400 I saved on the receiever to pay for the build/buy more ammo...
Federal Ordnance is okay if its already barreled and you have the headspaed bolt, if you are offered just the receiver, I would pass.
Poly-tech/norinco's are very good and would be a good basis for an upgrade, however the times I've seen just a receiver turn up they were selling for more than fulton receivers but less than an LRB. I personally would not pay $600 for a stripped receiver when poly's and norinco's turn up as complete rifles for around $700.
Armscorp are fultons and fultons are armscorps. Whichever you can get cheaper is the better buy. However, watch out for Sarco Inc's as the word on the street is that those are seconds.
I've heard some bad things on enterprise's M14 receivers so you may want to avoid those.
Also, I'll toss the disclaimer in that builds are not cheap. If its your first build please do consider buying a complete rifle (used SA's turn up for $1K to $1200, used Chinese rifles for $700 to $900 and Fed Ord's for $700 to $1K depending on parts), which in any case would be a lot more inexpensive than trying to assemble parts and have it built.