The books are about love and death. Love is a big quality determining the status of powerful. Voldemort could not love, Voldemort was beaten in the rank.

This leaves, based on the poll, Harry, Dumbledore, and Snape.

Most of what Harry could do, Snape and Dumbledore could usually do it better. Harry was not really "powerful". He was strong, brave, had all those qualities, sure. But I would not label him as powerful. He's further up in the rank than Voldemort, but still not powerful. He was not after power, he did things because he needed to, for the good of everyone.

That leaves Snape and Dumbledore.

A lot of people may disagree with me but Snape is a very powerful wizard. Not saying Dumbledore isn't, he is, but Snape is a way more powerful wizard in my opinion.

Dumbledore has a terrible past, he changed himself, his personality is powerful, awesome. He's wise, brave, etc, etc. He is the one wizard that Voldemort fears. He has that powerful impression on Voldemort.

Snape has a terrible past, he changed himself, his personality is powerful, awesome. He's wise (yeah, believe it or not), brave, etc, etc. He is the one wizard who acts on Voldemort's orders, becomes a spy, a double agent for both Voldemort and Dumbledore. The two wizards to fear of all time probably.

But there was one thing that kept him going, love. Dumbledore loved too, Dumbledore was all about love, that's also what makes him powerful, but unlike Snape, love was not what kept him going.

Snape loved Lily, there was a part of Lily within Harry, his eyes. Snape tried so hard to keep that part alive, so hard in fact, that he turned a spy for Dumbledore, he became a double agent for both Dumbledore and Voldemort. He ended up killing people, committing crime, doing terrible things, and dying for Harry.

That, in my opinion, is the reason Severus Snape is the more powerful wizard.