I love my various whistles.. some are brass, nickel, polymer...
But I think the four I reach for most often are
Black polymer Susato in C (because strong bright nimble notes with almost no effort. Easiest to get good sound among the 4 C's I've tried)
Black polymer Susato in Bb (clear but very warm tone, a little like recorder)
Jerry Freeman nickel Blackbird in high D (pretty warbly tone. notes are clear & easy to hop between notes without squeaking)
Dixon brass Trad in high D (because I like the chiffy 'hollow metal' tone, reminds me of temple bells or something)