Doflamingo for sure. He was a villain with some serious accomplishments under his belt. He was a warlord, he stole his own kingdom, and most importantly he was a driving force for Kaido's army and a lot of the turbulence in the New World. Doffy wanted nothing more than chaos and destruction in order to topple the world government, and he was making progress. He was also a big threat himself and no slouch when it comes to battle.
Akainu gets an honorable mention for trying to dress up his villainy as absolute justice and being a terrible person all around.
Akainu gets an honorable mention for trying to dress up his villainy as absolute justice and being a terrible person all around.

