From the 诗经·周南·汉广: 翘翘错薪，言刈其楚 - 'tall and tangled is the brushwood; let me cut the chastetree (楚).' 楚 was a tree that stood out above the rest, so 翘楚 came to mean 'one who stands head and shoulders above the crowd.'
register
Formal / literary. Common in news headlines, awards, and corporate copy ('行业翘楚' = industry leader). In casual speech you'd say 顶尖 (dǐngjiān) or 高手 (gāoshǒu).
