WordNet sense 1 (the highest level or degree attainable; the highest stage of development):

acme, elevation, height, peak, pinnacle, summit, superlative, tiptop, top

WordNet sense 2 (an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator):

line of longitude

WordNet sense 3 (being at the best stage of development):

prime

WordNet sense 4 (having reached full natural growth or development):

mature