Joffrey Baratheon was an arrogant, petty fool prone to random outbursts of anger, obsessed with the title of king he believed he inherited, even though he was little more than a puppet backed up by his grandfather Tywin. Joffrey never actually wielded a weapon, however, usually making a cowardly retreat when confronted with real danger.
Ramsay Bolton was sadistic and personally dangerous, capable of great violence and torment against people he had already captured, but he didn't have any true authority to back it up, and an impulsive, childish need to brutalize people for transient amusement with no thought to the long-term repercussions. Ramsay only knew random force and was in no way skilled at diplomacy, alienating potential allies.
Euron Greyjoy, meanwhile, isn't impulsive but capable of cunning long-term strategies, and unlike both of them, is actually capable of being extremely charming when the situation arises.
In many ways, Euron is the feared ruler that Joffrey and Ramsay wished they could be: genuinely dangerous and cunning, with a well-earned reputation, the authority to back it up, and a gift for long-term manipulation.