Kane and Taker definately, but Gangrel & The Brood were eerie too. The theme, entrance, and blood spitting and bloodbaths were pretty scary as a kid. Mankinds first entrance music was friggin creepy too... Sounds like the theme for the newer Texas Chainsaw movies lol.
I agree with those that said Papa Shango. He scared the crap out of me when I was a little kid. Just looking at his picture and listening to his music on the Super Wrestlemania game gave me creeps back then.
Kane is a good choice as well but I found him and Taker to be more cool than scary.
I remember Vader being after me in a nightmare I had in the early 90's. That also scared the crap out of me but I wasn't that scared of him outside of that one nightmare.
Papa Shango. Those gags they rigged up scared the crap out of children (and some adults) everywhere. It was brilliant. Plus Charles Wright was great at pulling it off with his strange facial expressions and over the top promos. Bravo.
Honorable mention to evil Doink the Clown before he turned into a kids/comedy act.