Okay Harry Potter fans, easy question: between Grindelwald and Voldemort, which evil sorcerer was more powerful?
Perhaps not such an easy question. And, unless J.K. decides to find a way to have the two of them duel each other, probably not one we will ever be able to answer definitively.
But, for the sake of the intellectual exercise, let's look at the evidence that Rowling does provide in the seven Harry Potter novels.
The only direct comparison made in the books appears in Rita Skeeter's scathing biography of Albus Dumbledore, which is excerpted extensively in The Deathly Hallows. In her book, Skeeter writes: "The name Grindelwald is justly famous: in a list of Most Dangerous Dark Wizards of All Time, he would miss out on the top spot only because You-Know-Who arrived, a generation later, to steal his crown."
In Skeeter's estimation, Voldemort was the more dangerous of the two. Does that mean he was more powerful as an individual wizard? Not necessarily: from my understanding, it means only that, in Skeeter's highly untrustworthy opinion, Voldemort posed more of a danger to the wizarding community than did Grindelwald.
But Skeeter does make an important point: in any comparison of the two evil wizards, Albus Dumbledore is a useful measuring stick. After all, Dumbledore faced both Grindelwald and Voldemort in battle.
Of Grindelwald, Dumbledore himself says this: "They say he feared me, and perhaps he did, but less, I think, than I did him...Not what he could do to me magically. I knew that we were evenly matched, perhaps that I was a shade more skilful."
And, of course, Dumbledore defeated Grindelwald in their fateful duel.
So Dumbledore believed and then later proved that he was slightly more skilful (powerful?) than Grindelwald.
What about Voldemort?
It is common knowledge that Voldemort feared Dumbledore. This is a fact repeated over and over again, by any number of knowledgeable persons, throughout the seven novels, beginning in the first chapter of the first novel when Professor McGonagall says, "Everyone knows you're the only one You-Know -- oh, all right, Voldemort -- was frightened of." At that point, of course, Dumbledore admits from the outset that "Voldemort had powers that I will never have".
It is also a fact that Voldemort never sought a direct confrontation with Dumbledore: their duel in the Ministry of Magic took place only because Voldemort arrived not knowing that Dumbledore was there already and ignored Bellatrix's attempts to warn him of Dumbledore's presence in the building; and Voldemort sent Draco Malfoy to kill Dumbledore in Book Six and assigned Snape the task of murdering the headmaster if Draco should fail, rather than confronting Dumbledore himself.
(Another interesting question: did Voldemort at that point know that Dumbledore had been injured by a curse? I doubt it. Even though Snape did not know about the Horcruxes, I would expect that he would withhold the key information about Dumbledore's injury from the Dark Lord. Imagine how differently the story would have played out if Snape had, in his ignorance, told Voldemort that Dumbledore had been injured cracking open ring? Would Voldemort have put two and two together and realised that Dumbledore was hunting Horcruxes?)
It also important to note that Bellatrix, one of Voldemort's most trusted followers, believes that it is imperative that she warn the Dark Lord that Dumbledore is in the Ministry, even though, at that very moment, she fears for her own life and faces severe punishment for her failure to obtain the prophecy. In fact, she tries not once but twice to warn him ("Master, you should know..." and later "But Master -- he is here -- he is below --"). This suggests that Bellatrix knows that Voldemort would not wish to meet Dumbledore in the Ministry, or at the very least might change his approach if he knew his former teacher was present.
And Voldemort's own reaction to Dumbledore's sudden appearance is also telling: "'What --?' cried Voldemort, staring around. And then he breathed, 'Dumbledore!'"
So Voldemort's is afraid when he discovers that he must face his former teacher.
In the duel that follows, Dumbledore's focus is split between fighting Voldemort, keeping Bellatrix trapped and protecting Harry, and yet he is still Voldemort's match. The battle ends in a draw, with the Dark Lord choosing to withdraw (first by possessing Harry in a vain attempt to get Dumbledore to kill the boy and then by fleeing the Ministry) rather than continue to fight Dumbledore.
It would appear from this evidence that Voldemort did indeed fear Dumbledore and that Dumbledore was the more powerful of the two.
I come to this conclusion even though Rowling writes, just before Voldemort possesses Harry: "For the first time, Dumbledore sounded frightened." From the context, I would argue that Dumbledore is not afraid of Voldemort at that point but for Harry. He recognises the threat to Harry and fears for his welfare.
I may be wrong, of course. Perhaps, in that moment, Dumbledore felt personally afraid of Voldemort. But even then, that does not prove that Voldemort was more powerful than Dumbledore but merely that, momentarily in a duel in which Dumbledore's attention was divided, Voldemort momentarily seemed to gain the upper hand.
So that leaves us with two Dark Wizards, both of whom feared Dumbledore and both of whom were unable to defeat him in a one-on-one duel. Grindelwald battled Dumbledore straight up and lost. Voldemort fought a distracted Dumbledore and withdrew.
It's not conclusive by any means but my gut tells me, based on the evidence in the novels, that Grindelwald would have bested Voldemort had the two ever duelled each other.