In the first book there is a discussion between <PERSON> and <PERSON> while they are waiting for <PERSON>. <PERSON> uses the name "<PERSON>" and <PERSON> cringes:
"My dear Professor, surely a sensible person like yourself can call him by his name? All this 'You-Know-Who' nonsense - for eleven years I have been trying to persuade people to call him by his proper name: <PERSON>." Professor <PERSON> flinched, but <PERSON>, who was unsticking two lemon drops, seemed not to notice. "It all gets so confusing if we keep saying 'You-Know-Who.' I have never seen any reason to be frightened of saying <PERSON>'s name."
"I know you haven't," said Professor <PERSON>, sounding half exasperated, half admiring. "But you're different. Everyone knows you're the only one You-Know- oh, all right, <PERSON>, was frightened of."
"You flatter me," said <PERSON> calmly. "<PERSON> had powers I will never have."
"Only because you're too - well - noble to use them."
I think <PERSON> lays it out right there, in the first chapter of the first book, that <PERSON> is the better wizard.
Also, consider this: when <PERSON> defeated <PERSON>, <PERSON> had the Elder Wand. <PERSON> defeated him in a straight up duel, not through trickery or subterfuge. That means <PERSON> was better than <PERSON> -- the most feared dark wizard until <PERSON> showed up -- even when he was at a disadvantage because <PERSON> had the most powerful wand in the world.
I imagine what it comes down to is that, in terms of raw magical power, <PERSON>, <PERSON> and <PERSON> were all equal. At some point, you have enough raw power to do whatever you want to. 773
It becomes a question of knowledge and how you use that power. <PERSON> -- of all three of them -- had the most complete understanding of magic. He knew how to use the dark magic that <PERSON> and <PERSON> employed -- but chose not to. Or else, knew of its existence and chose not to delve further than he had to. As I remember the quote in the first book -- and correct me if this is wrong -- <PERSON> says that <PERSON> has powers he "will never have". But not powers that he will never understand. That is a key difference. Conversely, <PERSON> understands and employs parts of magic that <PERSON> is completely unaware of. And even after he becomes aware of them, he doesn't truly understand them.
TL;DR In terms of raw power, I would guess they are close enough to equal. But <PERSON> has a much more complete understanding of magic than <PERSON>. 773