Probably ZENITH which basically means utmost.
apogee
apex
It is a word that contains two letter tiles worth 10 points each + a combination of one letter tile worth 3 points + four letter tiles worth 1 point apiece mostly vowels. And, one of the letter tiles with the highest points land on a double letter score and the word on a triple word score on the board.
A seven letter word beginning with C with a the fourth letter C is cracker.
allotmentamendmentargumentsassigningattachingattributeavailable
apogee
PEAK
This is not a "D" word but the best word in the whole of scrabble is "Mastery".
epitome
apex, acme
As. The shortest word beginning with the letter a is "a"
apex
Trough. The crest is the highest point of the wave while the trough is the lowest point.
Apex:From Chambers:apex noun (apexes or apices) 1 the highest point or tip. 2 geom the highest point of a plane or solid figure relative to some line or plane. 3 a climax or pinnacle. See also apical.ETYMOLOGY: 17c: Latin, meaning 'peak'.
The word "treetop" (long E) fits the bill.
The highest point of a fir is the treetop.
The highest point of a fir is the treetop.