Gender: Female Location: When in Doubt, Go to the Library.
I believe this is negatory.
He remarks that before Riddle knew he was a wizard, his powers were "surprisingly well-developed for such a young wizard..." p 276.
- and on p 501 Dumbledore says that as Voldemort Riddle had gone "further than anybody along the path that leads to immortality." Actually that's a quote of Voldemort's to his Death Eaters after his resurrection in book 4, but HBP is to hand and not GoF.
However, in SS/PS in a conversation between Dumbledore and McGonagall.
"You flatter me," said Dumbledore calmly. "Voldemort has powers I will never have."
"Only because you're too - well - noble to use them." p 11
So they have [at least] the same amount of power - Voldemort is just willing to do stuff that Dumbledore isn't. But Dumbledore is willing to do stuff that Voldemort isn't so there you go.
One could argue that Grindelwald is greater than Dumbledore - after all, Dumbledore did subscribe to Grindelwald's "greater good" anti-Muggle theory for a while. But then that was partially because Dumbledore loved Grindelwald, and we know that eventually Dumbledore defeated Grindelewald in a formal duel.
As far as Voldemort/Dumbledore dueling, the closest they get to it is in OotP, which is when Voldemort stops dueling to sort of take control of Harry. You could argue that Voldemort thought he was losing and so changed tactics... which is why I think Dumbledore is the most powerful out of the three.
Rowling has said that we'll get to see more about the relationship between Grindelwald and Voldemort, which I'm EXTREMELY excited to see.
So there's a Book Nerd's side.
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It does not do to dwell on dreams and forget to live.
So, JK Rowling has officially announced that Grindlewald is even more powerful than any of us thought.
He's a Seer:
"A Seer is a gifted wizard or witch who has the ability to see into the future with their Inner Eye. Seers predict prophecies, which are then recorded and stored in the Hall of Prophecy in the Ministry of Magic's Department of Mysteries. [...] Albus Dumbledore once told Harry Potter that seeing into the future is incredibly difficult because of the complexity of every single action and their consequences." [HP Wiki]
Ultimately however, the real danger is that if the dark wizard is a Seer as J.K. Rowling says, it also means he has a huge advantage over anyone else in the magical community (unless they also have a Seer fighting for them, which would be unlikely!) -moviepilot article, linked.
Which would mean that Dumbledore has proportionally more power, seeing as he beat Grindelwald in the first place.
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It does not do to dwell on dreams and forget to live.