Based on just pure raw skill, I’d say Grindelwald was probably more powerful and had a deeper knowledge of spells (probably overall magic). I think Voldemort was more cunning and ruthless though. Like a true snake his strikes were precise, quick and without mercy. You see that in their actions too. Grindelwald being more public, fighting while outnumbered more often etc. Meanwhile Voldemort was more in the shadows, assassinating folk, making horcruxes in case he did lose in a battle.

Yes, Grindelwald was doing his stuff for "the greater good", while Voldemort wanted to become immortal, and change the world into just people like him. In other words, Grindelwald was trying to get control of the world, while Voldemort's goals were to make the world his army.

Grindelwald was more powerful and direct, he's the better wizard, on par with Dumbledore at his prime, skilled and capable. Voldemort only seemed powerful because of his virtual immortality, which became a significant weakness when targeted. Grindelwald fights a dozen MACUSA wizards while Voldemort's duelling a fourth-year, Dumbledore at old age is fighting Voldemort while Grindelwald solos Aurors like its the back of his hand and duels Dumbledore, albeit he's defeated as Voldemort is due to Dumbledore being the most powerful of the three. Either way, both are cunning and evil, but one is more powerful while the other evades death by all costs.

Grindelwald was also a seer over Voldemort's parseltongue and legilimency, which is more powerful, that depends, but I'd personally have Voldemort's either way.

I think Voldemort was more powerful, though (my opinion)