This trigger set.
There's enough drop-in triggers for ARs out there to boggle the mind, everything from milspec(which range from 'not bad' to 'execrable', and from a few bucks to hundreds. Well, at the range the other day a guy had a AR in .300 Blackout that he'd built that had this trigger set in it. Comes with two hammer springs, one for 4.5#, one for 5.5#. He was very happy with it.
I got to put a few shots through it. This was left-handed from a rest(so no pressure on the right arm/shoulder), and I was impressed. Absolutely clean, no creep or drag. I really want to try this again when I can shoot more.