I really like penny whistles made of polymer (plastic). I think they sound warm and mellow.
Especially the ones in the key of B flat and C.  I also like that they won't get quite as hot or cold as metal whistles if left in my car for a while, or outside when camping.
Does anyone else here have a favorite polymer whistle?  My own favorite is my Susato Kildare black polymer in the key of C - it has a great strong but warm tone.