Valtor.

I like both but Valtor was a lot more successful as a villain. He is stronger than Ogron, because even though Ogron could nullify the Winx's attack (which is similar to what Valtor could do hrm) when they were Enchantix, as soon as they earned Believix and started making the people of Gardenia believing in fairies, they got substantially weaker. Ogron also has a company, with three other guys there to support him--whereas Valtor works mainly alone.

Valtor was created by the Ancestral Witches and is basically the darker version of the dragon fire, which, as we know from Bloom's overexposure, is the greatest and most powerful power in the magical universe. Valtor is strong after the Trix help him escape his ice prison, but he gets even more spells and effects by stealing magical scrolls throughout the realms. He manages to take over an entire planet (Andros) and this grip only falters after Tecna sacrifices herself, which is more than halfway through the series.

He blinds Layla after she just pisses him off, he takes one of Solaria's suns away and then uses Cassandra and Chimera as his puppets to use Radius, he turns Faragonda into a tree, he took over Cloudtower very easily and turned all of the witches into mindless zombies, he manipulates the crap outta Bloom and even makes her believe he killed her parents to just get away from them when he tries stealing from Alfea.

Even after everyone thinks he's dead, he makes a comeback and scares the fuck out of Helia, basically making the guy gives everyone a message that he isn't finished. He was one of the most successful villains in the show, that even the transformation on its own couldn't defeat them. The Winx had to go out of their way to find another power-up in order to become slightly stronger than him. He was an awesome wizard and one of the reasons why season three is my second favorite.

^
Couldn't agree more.

He is my favorite Winx Club villian.

He is my fav.