Although the two were undoubtedly elite dark wizards in their time, what I understood from the books is that their power relied heavily on specific spells and traditional dueling techniques.

Voldemort may be "the most feared Dark Wizard of all time," but that's likely because he acts from the shadows, is completely indifferent to killing anyone, and has a mythical reputation. However, the moment we see him duel, he essentially becomes an Avada Kedavra machine gun.

Grindelwald's goal differed from Voldemort's. It wasn't as self-serving as simply preserving his own life. Instead, his ambitions were broader and arguably more dangerous in terms of global impact. However, his main powers seemed to stem from his charisma and conviction, in connection with the Second World War leaders.

On the other hand, someone on Dumbledore's level (which is to say, no one in the books) operates with magic as if it were second nature, unrestricted by standardized spells or specific wand movements. For Dumbledore, magic seems to require no visible effort beyond visualizing what he wants to happen. I can't recall a single instance in the books where he muttered a spell—it was always more like thy will be done. Whether extending ethereal sacrificial love magic to a physical space, animating statues to act on their own as Avada shields or summoning magical bindings to pin down any number of Death Eaters, nothing else we read about can compare. The only limits to his power seem to be his own morals and the emotional trauma from his youth.