The internal parts that go inside the lower receiver are one of the most important factors in determining whether you have a good AR or just a so-so AR. In fact, when it comes to AR lowers it's about the only thing that matters. Your choice of buttstock matters I guess, but not so much.
I had good luck with a Rock River LPK that I bought through Joe Bob Outfittters. The trigger parts that make contact with each other were all highly polished and that resulted in a crisp, smooth as silk trigger pull. Everything else in the LPK seemed 1st rate and went together without any issues.