I'll be the first to admit that I haven't tried them all. Here's what (I think) I do know:
For a precision UPPER, the Vltor MUR is I believe a premier choice. Best one of which I'm aware, and has been for almost 20 years. Forged - stout as can be. Not a lightweight, either. Well engineered and well executed. Barrel extensions may require a tad of Locktite, since it is designed to work with anything. Guaranteed to rattle a little on most lowers, because it is purposely designed to work with ALL uppers. If yours is tight on your lower, it's due to tolerance stacking.
SanTan takes a different approach. For a precision upper/lower receiver set, SanTan is hard to beat. Let's go tight on tolerances!!! Don't try to slap your SanTan upper on anything but a SanTan lower. Plan on a solid 30 seconds with your heat gun on high to get that Proof Research CFW barrel extension to fit. The now discontinued Young MFG National Match bolt carriers would not even fit inside the early SanTan receivers.
Another thing with SanTan - The anodizing so so good that it makes the other aluminum parts on your gun look kinda cheap.
Are you still making barrels with kac's intermediate gas length?