Top brands, in terms of quality, reliability, and low likelihood of failures I recommend the following.
Noveske, Daniel Defense, BCM, Colt, Colt Canada.
Specifically, id recommend a Daniel Defense BCG because they're just as good as the other ones but can usually be found much easier. That being said, you'd probably get equal results out of something lower on the totom pole like DPMS, stag, spikes etc.
For barrel, that's gonna be the money maker, so don't cheap out. I presume you're going for Non Res with the MacDef receiver so that may pose a challenge finding something over 18.6 but if you can, I'd recommend a noveske barrel. Pricey, but highly regarded for their quality and accuracy. Second to that I believe BCM offers a 20" barrel but I can't be certain.
Another great option would be Faxon barrels. More cost effective but if you're shooting more "minute of bad guy" vs MOA, they're a great choice that won't break the bank.