Just going by straight up coolness, it's Soundwave by a longshot.
I used to sit next to the TV with my tape recorder when watching Transformers, just so I could get Soundwave's voice on tape so I could play it back later.
Skeletor looks bad-ass, but his voice on the cartoon was such a total opposite of what you would expect, which is part of the Saturday Morning Cartoon charm about Masters of the Universe I guess.
Fun Fact: Starscream & Cobra Commander were voiced by the same actor. Cobra Commander with the face shield beats hooded Cobra Commander any day of the week. I always thought Destro was way cooler anyway.