Who's stronger? Voldemort or Grindelwald?

Asked by @PrinceLeviathan

Personally I think Grindelwald is stronger, for several reasons.

1. He wasn't destroyed/killed by a teenage boy and actually lived to be 115.

2. He didn't even know what horcruxes were and he didn't need them in order to live and become a powerful villian.

3. He dueled Dumbledore in what was so called a legendary duel. Even though he didn't win he did duel one of the most powerful wizards and that shows something about his strength and power.

4. Voldemort needed helped from many others. (Examples: Wormtail, Lucius, Snape, etc.) Whereas Grindelwald didn't really need help from anyone. Of course he did get help from some people but not as many people as Voldemort did.

5. I'm pretty sure Grindlewald did far worse things than Voldemort. Voldemort tried to take over a school and couldn't even do that... that shows how weak and powerless he is.