Voldemort ultimately gained more power, but quotes from the Wizarding World franchise might prove that Grindelwald was the greater antagonist.
The Fantastic Beasts movies have opened up audiences to another villain of the Harry Potter universe. Grindelwald's goals are similar to Voldemort's: wizards ruling over the muggles and no longer needing to hide. However, the motivations for this goal, as well as the methods used to go about achieving it, differ greatly from villain to villain.
While both Grindelwald and Voldemort use dark magic, murder, and social manipulation to rise to power, and both were ultimately defeated, it can be difficult to decide which of them was the worse villain. However, different quotes from the Harry Potter books and movies, as well as the Fantastic Beasts films suggest that Grindelwald may have been the one to be reckoned with.
This quote perfectly sums up Dumbledore's feelings towards Grindelwald, which is what made him so dangerous. When they were young, as Dumbledore stated in this quote, he and Grindelwald both felt that Muggles would be better off if they were led by wizards. They felt that the abilities of wizards gave them the right to rule and that muggles needed to be put in their place.
Such conviction in a belief can be very dangerous. Grindelwald knew that he was powerful, and believed that that was what made him and other wizards specially qualified to rule Muggles. He didn't just believe in himself but in all of wizarding kind.
In comparison, Voldemort's beliefs were far more simple. He wanted power for himself. While he saw his status in the wizarding world as what made him special compared to the muggle children he was raised tormenting, he had delusions of grandeur about how he compared to other wizards as well.
Someone fighting only for themselves is far less terrifying than someone who is fighting for a world that they truly believe in. But Voldemort only wanted a world that he could control.
Grindelwald knew that his power would not be enough to achieve all that he wanted. As powerful a wizard as he was, he knew that he would need ranks of loyal supporters, and he achieved this by making connections.
He used his understanding of people to convince them that his way was the way that would give them everything they wanted. And it worked to great effect. Voldemort could never have made these connections as he did not understand other human beings.
Voldemort had leagues of supporters, but he never believed in any of them. In his opinion, no one would ever be as clever as him. He underestimated the intelligence of everyone, calling the likes of Harry a fool for even having hope.
He felt that this quality made him stronger, but by underestimating those he believed to be his followers, as well as his enemies, Voldemort ultimately lost his victory.
Grindelwald saw his fellows living in a world where they had to keep silent and hide their potential from those he saw as weaker than them. He also knew their power was all they needed to grant themselves a different world.
Vengeance is a strong motivator. When someone believes that their actions are for vengeance, there are very few lines that they won't cross, making them unpredictable. When a demagogue like Grindelwald uses this cry for vengeance to arm a society of hurt people, the outcome can be catastrophic.
Voldemort sought to extend his life. He wanted power, and he wanted to live forever unchallenged. This was why he made various decisions, such as creating Horcruxes and attacking the infant Harry Potter.
When someone is seeking personal gain, they become predictable. All of Voldemort's actions were anticipated by Dumbledore, who was able to understand exactly what someone fighting for survival with no regard for others would do. While Voldemort still caused a lot of damage, he ultimately took himself down with his quest for self-preservation.
Grindelwald dove deep into dark magic to discover the ways he could gain more power. He researched every avenue and gained a mastery of many. He understood the powers that come to play in things like the Elder Wand, and even the powers of love, having experienced such emotions for himself.
This holistic understanding of magic allowed him to understand his enemies, and cause chaos in even more ways. Grindelwald was killed by Voldemort in the Harry Potter series, but the Fantastic Beasts character laughed in Voldemort's face, knowing he was doomed to fail.
Voldemort felt that some areas of magic were a waste of time. He had never experienced love, and could therefore have no understanding about how this ancient magic could ever hurt him. This ended up being a weakness that was pointed out by Dumbledore.
If Voldemort had not taken certain forms of magic for granted, he would have better understood the magic of love and wand lore and might have won his final battle in Deathly Hallows.
Grindelwald was flawed, but he was entirely human. He was intelligent, cunning, and talented, traits that initially lured Dumbledore in. While Dumbledore knew that Grindelwald had a darkness in his soul that propelled their mission forward, Dumbledore also saw good in him too.
While Dumbledore eventually was forced to see the repercussions of allowing such darkness to exist, he was initially taken in wholeheartedly by Grindelwald, a sign that he was someone to be feared.
Voldemort was equally as talented as Grindelwald. He could be charming when he wanted to, and all the teachers of Hogwarts were taken in by him, but not Dumbledore. Dumbledore, who was a powerful wizard full of love and feeling, could easily sense that Voldemort could not be trusted.
The fact that a powerful Wizard such as Dumbledore could be fooled by Grindelwald, and not Voldemort, shows which of the two wizards was the real threat.
