Grindelwald wielded the Elder Wand, was easily capable of fending dozens of Aurors at once and casually unleashed a spell that almost destroyed Paris. Not to mention that Dumbledore is specifically confronted and told that nobody other than him can match up to Grindelwald.

 

I don't know if Grindelwald is genuinely a 'Dumbledore tier' wizard (although I absolutely think that he is). I'm saying that Dumbledore's public legend and perception of greatness is based in no small part on him beating Grindelwald.

As an example, his chocolate frog card biography:

"Considered by many the greatest wizard of modern times, Dumbledore is particularly famous for his defeat of the Dark Wizard Grindelwald in 1945, for the discovery of the twelve uses of dragon’s blood, and his work on alchemy with his partner, Nicolas Flamel. Professor Dumbledore enjoys chamber music and ten-pin bowling.[1]"

 

This discussion - and forgive me if I missed it somewhere in the last two pages - also needs to take into account intent. I think JKR even accounted for this a little when she determined that using an Unforgivable Curse meant you had to mean it.

Grindelwald, Dumbledore, Voldemort = all supremely powerful wizards, but also vastly intelligent men.

A distinction needs to be drawn between witches and wizards willing to use darker magic - Horcrux, Killing Curse, etc - and those that would never, but also understand through their innate intelligence that doing so gives them an edge.

Following my logic to it's conclusion, Hermione would be the darkest Dark Lord ever known, if not for a grounding in morality, which is what we saw in Harry. A competent wizard, sassy and rowdy, but lazy. His intent, however, wasn't aligned with seeking power or privilege. One could argue Dumbledore, Grindelwald, Voldemort did use their intelligence to seek power, and were thus flawed.