I think Big Mom and Kaido are still worse than Doflamingo.
Both pulled some crazy shit in which Doflamingo looks like a saint in comparison. Nevertheless, if Kaido, Big Mom and Imu weren't there, I'd choose Doflamingo as well.
Garling has a potential spot as well.
Yeah, BM and Kaido have some bad shit behind their belt but I still think Doflamingo tops them, considering his strength and his organization compared to their strength and influence. Afaik, he was named the champion of evil. He could have been the secret end boss of the story if he was stronger.
BM and Kaido are simply natural disasters who are from birth superior to others so they see no value in others who are inferior. Meanwhile, Doflamingo is a sick psychopath who clearly enjoys the shit he is doing. While Kaido and BM see others as inferior, Doflamingo sees them as his potential slaves that could have been his thanks to his birthright that was taken away...
He does have a soft spot for members of the DonQuixote family but otherwise, he is as ruthless and menacing as it gets.
In that regard, Big Mom is the sickest, even Kaido has a soft spot for his commanders while Big Mom treats even Katakuri as "lowly son of mine"...