Sean Malone of Gordian Not, Cynic , and Liquid Tension Experiment is pretty much a beast. If you want to hear a good example of a fretless used in a metal band, Cynic is probably one of the best examples for thatCynic is very experimental, Gordian Knot is more accessible, as is LTE, whose first album features a 27 minute song. Pretty crazy, very progressive stuff.
I just mentioned this tune elsewhere but Rivers Dancing by Gordian Knot has one of my favorite fretless sections ever. The little solo is deceptively easy to play and sounds unbelievably emotive.