Buy from those companies which have a reputation for consistent specs. There really is no "best", it comes down to what you personally want and/or can afford. Truly, you'll find one mans treasure is another mans trash, and the threads are here to prove it.
I've built AR's on Bushmaster, Eagle/ArmaLite, Colt and Rock River Arms lowers. Have had excellent results from all, and all were perfectly in spec. If there were no identifiable marks on the lowers, I would have NO IDEA which was which (except the Colt, which had different size trigger pins-AHHHHHHHHHH, Colt!). Fit and finish were all perfect. And all those for whom the specific weapons were built were pleased. I've never had ONE complaint.
So, dude, it's entirely up to you and your wallet. Which is basically a truism for everything else in your life.
Prepare for the inevitable "Colt sucks, Bushmaster sucks, DPMS sucks and I'll only use XYZ lowers 'cuz that's what the MILITARY uses and I've got a cuzzin twice removed who knows a Special Forces SEALS Green Beret Marine Recon sniper guy in Iraq who sez he used his XYZ built M16 to launch a new guided weapon which single handedly wiped out the Taliban, but he can't talk about it...", etc., etc., you'll see it all.
Good friggin luck