I'm currently working on building a Rock River Arms carbine. Picked up the lower at a show and ordered the lower parts and stock directly from Rock River.
"Best" is a relative thing. To me, RRA was best because I could buy an in spec lower for a little over $120 at the show. There were no ABC lowers to be had, and had there been, they would have cost a lot more. And, people over on AR15.com say good things about RRA, so I went with them. Eventually, I'll use a RRA upper (just can't decide which one); I think mixing brands between upper and lowers, while lots of times it works, is generally bad juju. YMMV
Be careful about where you buy your parts. For my money, it's better to pay a little more and get parts from a reliable supplier, like RRA, Bushmaster, Armalight, Colt, etc., than to take a chance on what you're getting from one of the parts houses. If you go over to AR15.com and ask who not to buy parts from, you will see a pattern develop very quickly.
I would also strongly suggest the book "The AR-15 Complete Assembly Guide" by Walt Kuleck and Clint McKee. It's available from scott-duff.com and worth every penny to a first time builder. The pictures themselves are worth every cent and the tips and advice will keep you from making mistakes. Trust me on this, you will thank me later.