Ekaterina II - not Russian, not Orthodox-born and with no claim to the throne, and manages to depose her legitimate husband;
Henry IV, Edward IV and Richard III (sorry, but Henry VII doesn't count as a usurper to me, since he later legitimized it by marry Elizabeth of York);
Oliver Cromwell - don't really like him enough for him to be a favorite, but the man ruled the UK with more power than the king for over a decade;
Philippe Égalite, duc d'Orléans, for his hope that he'd be chosen as king to replace Louis XVI. He gets my respect for sheer gall, even if I detest his betrayal;
Karl VII, Holy Roman Emperor and King of Bohemia, married to a younger daughter and still manages to get himself elected ahead of his brother-in-law and cousins-in-law;
Frederick V of the Palatinate - because, reasons
The Bonapartes
The Coburgs - esp. Ferdinand of Kohary and Prince Albert (and Leopold I, too, since he would most likely have done the same to Princess Charlotte).