WordNet sense 1 (the period of greatest prosperity or productivity):

bloom, blossom, efflorescence, flower, flush, heyday, peak

WordNet sense 2 (the time of maturity when power and vigor are greatest):

prime of life

WordNet sense 3 (a number that has no factor but itself and 1):

prime quantity

WordNet sense 4 (of superior grade):

choice, prize, quality, select

WordNet sense 5 (of high or superior quality or performance):

superior

WordNet sense 6 (being at the best stage of development):

meridian

WordNet sense 7 (cover with a primer; apply a primer to):

ground, undercoat

WordNet sense 8 (first in rank or degree):

premier

WordNet sense 9 (preceding all others in time or space or degree):

first