Since there isn’t enough information for Grindlewald yet, this kind of has to go to Voldemort. However, I think a quote from Dumbledore in the HBP, “Did I know I just met the most dangerous Dark Wizard of all time? No.” Seeing as we know Dumbledore has faced off against both of these individuals, he can give an accurate sense of what these two characters would be like in a fight. So I would honestly say Voldemort would be Grindlewald, in a very very very close fight.