1) Acme 2) Copestone 1) Acme of achievement 2) Copestone 3) Crowning achievement 4) Crowning event 5) Finishing touch 6) Most notable achievement 7) Touch 8) Turret topper 
• (n.) A fossil echinus of the genus Cannulus; -- so called from its supposed resemblance to a cap.
Cap'stone` noun (Paleon.) A fossil echinus of the genus Cannulus ; -- so called from its supposed resemblance to a cap. 
The crowning stone of a structure; differing from capital in that it is not a supporting member.
(from cope, Latin capa), consists of the capping or covering of a wall. A splayed or wedge coping slopes in a single direction; a saddle coping slopes to either side of a central high point.
[n] - a stone that forms the top of wall or building
Coping on top of a wall.
copestone noun a stone that forms the top of wall or building