I just checked and Pottermore.com, the official website for all things Harry Potter, started and endorsed by Rowling herself, describes Voldemort as "the most powerful dark wizard of all time" on his bio.
Between that and all the evidence on screen and on the page, that settles the debate. The only way Grindelwald wins is if he gets lucky. And Grindelwald is called dark wizard too.
The only thing Grindelwald did that Voldemort didn't was have more conquered land under his belt. But that's only because Voldemort started with the hardest country first. Also,that period was different and it was a period of war.
Grindelwald only got as far as he did because Dumbledore wouldn't face him until there duel, while he actively worked against Voldemort during his reign (and from what some characters said it sounded like Voldemort was winning) and the book doesn't say that Grindelwald conquered Europe.
Also we don't know how powerful Dumbledore was in 1945, for all we know he might have not been as experienced then as when he fought Voldemort while using the elder wand against him and protecting Harry wasn't really an issue since Voldemort didn't really pay him any attention when Dumbledore arrived.
Grindelwald used the wand against Dumbledore and Dumbledore used the wand against Voldemort.
Dumbledore>Voldemort>Grindelwald.
This nonsense Grindelwald wank needs to end.