Aww...man this one is tough! Dewgong is out of the running because although it looks nice, it's not necessarily that great [although a Hydration Dewgong would team up great with my Rain Dance Ludicolo :) ]
Slowking's a bit slow, but has a cool design, evolves via King's Rock just like Politoed, which automatically makes it awesome...and also, Slowpoke have their own well! Plus I do like seeing Slowking following me around in HeartGold.
Scrafty on the other hand is really tough, pretty sweet design, one of the ones in Black that I genuinely do like, rather than looking for similarities with previous pokémon. The Dark / Fighting combo is pretty good too...nothing beats going out against a similar-levelled Scrafty and both using Hi Jump Kick :)
Overall I'm probably going to go Slowking just because it looks like it'll get less votes, despite it being no less cool than Scrafty.