Who would have won in a war between Grindelwald and Voldemort, when both where at the top of their power?

In a war? Meaning between their respective armies? Or in a 1v1 battle? Those are 2 different answers.

Lord Voldemort was a much more magically powerful and skilled wizard than Gellert Grindelwald.

In a one vs. one duel, it seems unlikely that Grindelwald could ever hope to win. Unless he managed to outsmart Voldemort in some way, Voldemort would undoubtedly defeat him. But if you mean who’d win in a war between their respective armies, then:

Gellert Grindelwald is arguably the most politically powerful, influential and commanding person ever to live in the wizarding world.

Grindelwald literally commanded legions. Unlike Voldemort, who ruled through fear, Grindelwald ruled through persuasion. He had many thousands of followers that spanned across the globe and he was the face of an international revolution. Voldemort had a few hundred followers at most, and we know that most of them were only his followers because they feared him; they did not follow him out of any real loyalty. Voldemort’s forces would be completely overwhelmed by Grindelwald’s armies, and as powerful as Voldemort is as an individual, it’s doubtful he alone can take on an army of wizards and witches. It would be the equivalent of Leonidas and his 300 soldiers against Hitler and the entire Third Reich. In a fistfight, Leonidas could destroy Hitler just like Voldemort could defeat Grindelwald, but the spartan forces would be no match for the Nazi regime.