How many letters does the top answer for “of the highest quality” have?

The most common answer is ace (3 letters).

Crossword clue: of the highest quality

4 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

ace 3 letters wn-def

the smallest whole number or a numeral representing this number

crack 5 letters wn-def

become fractured; break or crack on the surface only

tiptop 6 letters wn-def

the extreme top or summit

topnotch 8 letters wn-def

of the highest quality

not of the highest quality or sophistication 50% similar → bush

very good; of the highest quality 50% similar → excellent, fantabulous