for some nice, custom and semi-custom guns from a top name 'smith at reasonable prices.
As far as barrels... they are about like any other gun, in that there isn't a 'best' one for them. Lots of HP shooters love Kreigers, others think Pac-Nors are just fine, and most factory guns shoot some pretty darn small groups w/ inexpensive Wilson barrels.
I'd suggest you head on over to Sinclair International and order some of their books on ARs. One thing to keep in mind, relative to some of the comments on Colts in those books: By and large they are awfully expensive to build a gun off of compared to the high quality options from Bushmaster and Rock River available these days. By the time you fork over the $$$ for a stock rifle from Colt, or some of the whiz-bang stuff from places like Fulton Armory, JP Enterprises, you could easily pay for a space-gun match upper from somebody like John Holliger @ White Oak Armament, buy a RRA lower separately and be done for under $900, and it'd be pretty much top-shelf.