All of the players mentioned here are excellent (and, unfortunately, underrated). However, my ultimate favorite underrated jazz trumpeter would be Italian virtuoso Fabrizio Bosso. I have 2 of his albums, and both had to be specially imported (Rome After Midnight, and Trumpet Legacy, feat. Flavio Boltro, another underrated great). There are some great audio clips on his website:
[ Fabrizio Bosso - Official Web Site ]