I've used Daniel Defense, Delton, RRA, RB Precision, DPMS, J&T/Doublestar, and CMMG. I haven't had any trouble with any of them. RRA and RB Precision are probably the easiest to install due to the rolled roll pins instead of split. CMMG is a bit odd because of the plastic detents, but they worked fine.
My DD hammer was notched but some guys have gotten kits with standard hammers.
If your on a budget get the DPMS, Daniel Defense has the nicest finish but one of the heaviest trigger pull, very smooth but heavy.
I'm planning on using Spikes, Bushmaster, and Stag for my next three kits.
The companys on my stay away from list are Model 1 Sales, Blackthorne, and Olympic.
ETA: one thing to remember is if you want to run a .22 kit you need an unnotched hammer like the DPMS.