The two Water/Dark types. Both of them have a great physical attack stat and a decent Special attack. They are actually okay in competitive battling too, largely due to speed boosts and huge attack. Sharpedo has Speed Boost from the Dream World to skyrocket its speed, while Crawdaunt has the less reliable but still good Dragon Dance. Crawdaunt also gets Adaptability to make it hit even harder than it already does.
-Which one has the best design?
-Which one would win in a battle?
-Which one is the most useful?
I personally prefer Sharpedo by far. I absolutely love its design, and personally I find it more useful due to its much more reliable speed boosts. Im not sure which one would win in a fight, however...
Crawdaunt is still cool tho.