go with a Rock River. They have the best QC. If you want a really good one, upgrade to their two stage trigger, and you will have a combo that is hard to beat.
I second the vote for Rock River. I have two builds. One was using a DPMS LPK, and the other using a RRA LPK. You can really tell the difference, visually, and when you pull the trigger.
Built a couple lowers this weekend. I've used RRA, DPMS and Double Star LPKs. The double star I picked up at the last 1500. I liked the RRA the best with the Double Star a close second. Mine came with a couple extra springs in case you shoot them across the room. The DPMS was ok and the only reason I bought it was because it was really cheap ($53).