It is my belief that, in terms of raw magical power, Dumbledore, Grindelwald and Voldemort are indeed close to equal, being 17 years old Tom Marvolo Riddle slightly more powerful than 17-year-old Albus Dumbledore, and Dumbledore slightly more powerful than a 17-year-old Grindelwald.
In terms of general knowledge of magic, in terms of academic brightness, I would say it was Dumbledore, Grindelwald and Voldemort. The first knew the entire spectrum of magic but chose not to delve into the Dark Arts. The latter two knew quite a lot of magic, and were specialists in the Dark Arts. Voldemort, being awfully ignorant of the consequences chose to delve further than anybody else. But I don't think of him as a creative genius. The only two things he invented were the Despair Potion to protect the locket and the Dark Mark. He didn't invent the unforgivable curses, he was just the best practitioner of them. He didn't invent Inferi, or horcruxes. He just made better ones.
Dumbledore thinks that Voldemort knows more magic than any of the Wizarding world. I have my doubts. He certainly knows many potions, many spells, and he performs magic with outstanding performances with extraordinary might. But he seems to fail to fully grasp what he's doing. He underwent body transformations to become stronger (we see this when he uses Harry's blood to make his new body indestructible - like he believed Harry's to be). And drank unicorn blood to that end also.
In terms of dueling skills, I think that it's clear Dumbledore was more skillful than Grindelwald; now, if he didn't have the Elder Wand, perhaps - and this is big perhaps - Voldemort might have defeated him in the Ministry. This is big maybe. Because we see that Dumbledore was not really trying to defeat him, rather trying to protect Harry, himself, and keep Voldemort from leaving until Fudge could arrive. So when he tried to engulf Voldy in the water bubble, the spell might have not be strong enough hold him for more than a second, and Voldy would have hit the killing curse on Dumbledore. But this is big maybe. We all saw how clever and how quick Dumby was. Because he had the Elder Wand, it's possible that he was able to delay him just enough for Fudge.
Regarding the powers he said he will never have, I believe he meant Parseltongue, natural born Legilimency, and possession. You can learn Legilimency (like he did), and learn to understand Parseltongue (like he did), and even say a few words like Ron did, but Voldemort inherited those powers from his mother.
A good comparison would be Darth Vader, Darth Sidious and Yoda. Yoda being Dumby, Sidious being Grindy, and Vader being Voldy. Perhaps, the last two switched given Sidious mastered abilities Vader never did, while Vader was much more powerful. It is said that, at their time, each one of them was considered to be the brightest student to have ever set foot in their school. Though I find it hard to believe Dumbledore to be brighter than Merlin (the Prince of Charms - many of the Transfiguration spells and charms used today were invented by him) and Riddle to be brighter than Dumbledore.
I don't think, however, that Voldemort feared Dumbledore because of his skills as duelist, or magical might. I think it's gravely subjective to him that Dumbledore saw who he was at the age of 10/11, and that he was never able to fool Dumbledore. Also that, like him, he was very well informed of everything that was happening. I believe he disappeared after killing Hepzibath Smith precisely because he thought Dumbledore might have connected the killings of Little Hangleton (the Riddle family) in the previous year, with the death of Smith. Morfin Gaunt complained that his ring was stolen, and Smith's Cup and locket she purchased from Borgin & Burkes had disappeared according to her house-elf. The same way he meddled and altered his uncle's memory, and the elf's memory, Dumbledore could have denounced him and put the entire Ministry, plus himself after him; he was still carrying his horcruxes with him, because he hadn't engineered enough protection to safeguard them.
To sum up: like Dumbledore feared Grindelwald for his secrets, Voldemort feared Dumbledore because he could breach into his secrets. Besides, Dumbledore was an exceptionally brave man, he faced his fear of Grindelwald and took him on in his territory. He faced Voldemort all along doing wonders at great personal cost - a true Gryffindor. He was willing to take on Fudge and the entire Auror office alone, while Voldemort only personally attacked Hogwarts with an army of followers, leaving the most important part of the battle for his Death Eaters, his giants, his dementors, his Ministry employees, his snatchers.
Therefore, Dumbledore was by far the better wizard. Not to mention that he set the chess play in motion so that Harry could finish the job; stupidly Voldemort took the bait, and played along, just as Dumbledore had foreseen, and he was beaten as a result.