It's really tough because they're all very powerful. They also each ahve different strengths and weaknesses. Plus, Harry still had plenty of time till he reaches Dumbledore and Voldemort's age's so by then he could be more powerful.
But based on what happens in the books, i'm going to go with Dumbledore. Overall, I think he's more powerful and knows how to use it.

Voldemort and Dumbledore of course!!!Harry is not even close to their powers!!!

The facts speak for themselves, whether he is honestly more powerful than Dumbledore/Voldemort is questionable, but it stands to reason that by the end of the series Harry was the one left standing. If only by luck and illogical perseverance (although awesome!) Harry is the most powerful. Unless you mean on ability alone, because Dumbaldores the S**t then :)

Tough question... Harry was only powerful through his connection with Voldemort and Voldemort was powerful because of his rage. The most educated, wise and talented, powerful one must be Dumbledore then.

dumbledore

They're all powerful wizards... but...
Harry was powerful because of the connection with Voldemort's wand, and if that hadn't existed, Harry would have been dead three times over.

Voldemort is powerful, but he is weak in other ways. He has nothing to fight for, and if he did, he would have been WAY more powerful.

But Dumbledore... now, that's a guy with the full package. He was fighting for the greater good, and he was the ONLY wizard V actually feared. Moreover, his superiority over V was not because of a connection, it was because of his sheer skill and wisdom.

i totally agree with @lovedraco123!!
i mean harry as brave and courageous (we still at least have to give him credit for standing up to voldy over here and also not his fault that Vold gave him powers) as he may be. the only reason he was powerful because voldy accidentally gave him some power.
voldy is just weak.
i'm sorry is used voldy cause his name is just to long to spell over and over again. -