I STILL say “front sight”...
Read Ayoob. I think the actual sequence, IIRC, is 1)COVER, 2)Concealment, 3) then engage. Clear your mind of everything except front sight. Ayoob wrote that when they studied quite a number of LEO shootings many of the skilled shooters had struck the assailant’s weapon or weapon hand. After interviews they realized that THAT was what the guy was looking at. Like you say, Laz, center mass, front sight.
Good points, and my thoughts were remiss.