Would have been helpful to have made a list of unridable dragons to start with so those who have only seen the movie wouldn't have to go by trial and error

If the Skrill interest you then maybe a Flightmare? There seem to only be three [url=http://howtotrainyourdragon.wikia.com/wiki/Category:Strike_class]'strike-classs'[/url] dragons: Flightmare, Skrill, and Night Fury.

It's rather powerful according to the wiki, but so is a Night Fury