1. first-class; first-rate; top-flight; top-ranking; leading; eminent; top
どれをとっても一流といっていい。Any of them can be called first-class.
2. characteristic (of); peculiar (to); unique (to)
それは彼一流の冗談だった。That was one of his typical jokes.
3. school (e.g. of flower arrangement)
4. one flag; one banner; one streamer
also written as 一旒