To make things fair.
Viper has lots of S Class~
There's
Sairis, Nathan, Maitsuki, Black Mamba, Brad Li and lastly, Marduk the Wizard Saint, trump card of Viper.

Harpy has Revan, Aria, Merrian.

Harpy lacks it. Yeah, he S Class.
Accepted.

EDIT: To add to the hype train.
Howling Pack Elite of Lunar Wolf are all S Class, f@ck it. At the end of the day, it's just a title. Except for Oram xD 

MORE EDIT: f@ck it. I'll accept 1 more S Class for Lunar Wolf. Then that's it....