Toss-up between UMK3 Reptile (which translated fairly well to MK9 Reptile admittedly), Deception's Havik, and MK4 Quan Chi. I always liked that Reptile was a little different from Scorp and Sub (teeth in the mask) and the green acid balls were stylistically really unique to fighting games at the time. Havik had my two favorite 3d-gen stances and a slew of awesome low attacks. Quan chi's original skydrop was absurd, and Leg Rip gets my vote for most badass fatality in MK history.