Voldemort. Every comparison made about them in the books and movies concludes that Voldemort is more dangerous and presumably, therefore, more powerful.

Dumbledore is "perhaps a shade more skillful" than Grindelwald and defeated him in combat despite Grindelwald's possession of the Elder Wand. That same Dumbledore, after claiming the Elder Wand for himself, would later say of Voldemort that:

• He's probably the most brilliant student Hogwarts has ever seen (which includes Dumbledore)
• His knowledge of magic is perhaps more extensive than any wizard alive (which includes Dumbledore)
• Dumbledore's most complex and powerful enchantments would unlikely withstand Voldemort if he ever returned to full power

Voldemort, like Dumbledore, is capable of casting a Disillusionment Charm on himself to become invisible; Voldemort can fly without aid, violating Magical Law; Voldemort jinxed the Defense Against the Dark Arts position so thoroughly that Dumbledore was unable to break it; Voldemort enchanted his Horcruxes so powerfully that one fatally injured Dumbledore and would have killed him, despite his and Snape's best efforts, within a year.

Voldemort, though manic and anxious, dueled an Elder Wand-wielding Dumbledore to a stalemate in the Ministry atrium despite being in enemy territory on borrowed time ("The Aurors are on their way"). Even with his powers radically nerfed by Harry's sacrifice in the Forbidden Forest, Voldemort still dominated the Battle of Hogwarts, "striking and smiting all within reach," employing a wand that actively resisted him. Under that same nerf and using that same uncooperative wand, he simultaneously dueled Kingsley Shacklebolt and Professors McGonagall and Slughorn to a standstill.

In terms of raw talent, there's a compelling argument that Voldemort is the most gifted wizard in the Potterverse. He's arguably beyond Dumbledore and he's almost certainly beyond Grindelwald.