Optimum (4)
(Other definitions for acme that I've seen before include "Top or highest point" , "Height of excellence" , "'Highest point, apex (4)'" , "Height of perfection" , "Summit of achievement" .)
'optimum' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for best that I've seen before include "Superlatively good" , "Worst -- as good as can be" , "12 21[FOOTBALLER" , "Most advantageous" , "erstwhile drummer" .)