I second the nomination for the original Cybershark (hammerhead shark) or Sharp Edge (sawshark) -- between those two I personally prefer Sharp Edge - but Cybershark is fine too. Those would be the most realistic looking shark moded Transformers. All other sharkformers are cybernetic or robotic sharks (or in the case of Hammerstrike, "Technorganic").

My least favourite sharkformer would have to be TM2 Cybershark, who was the only one that was available in Australia too. (oh, and Energon Sharkticon too if you wanna include him (he transforms into a sharklike boat, but not an actual shark)