Mine is definitely Snape. One of the most powerful characters in Harry Potter and also the smartest.
Unlike in the movie, the book says that he and Voldemort were the only ones capable of flying without a broom. Voldemort had created the spell, but Snape was the only one who could reproduce it.
Snape also knew about the prophecy of the child who would be born in July and would defeat Voldemort. But no one knows why he didn’t go there to protect his beloved, only going there when she was already dead.
After Snape, I also like Bellatrix Lestrange.
She scares everyone and I think her power is underestimated.
While fleeing the Department of Mysteries, she managed to block a spell from Dumbledore, when he had the Elder Wand.
And everyone saw what she did to Neville’s parents with a simple Crucio.