Lol Abyss.. are you kidding me? IMO, that is probably one of the worst monster gimmicks ever, he isn't even intmidating at all pretty much a Kane/Mankind rip off yet looks way more goofy looking, and I think he absolutely blows in the ring and can't sell shit (see Angle vs. Abyss matches, their first match he had no idea how to sell the Ankle Lock...), but whatever it's your opinion I guess.
I would have to go with Vader, he was the shit back in the day, was awesome on the mic and original. If not Vader, then Kane on his debut, where he was intimidating and scary looking as hell. But nowadays, nothing like what he use to be. =(