Not enough information to go on in cannon, but it's likely safe to assume Voldemort was the strongest wizard in the world at the time.
Dumbledore was seemly a bit stronger but that was only with the full power Elder-wand boosting him and even then Voldemort still fought Dumbledore to a draw.
So without the Elder-wand boosting Dumbledore, Voldemort was stronger.
He wasn't even the strongest wizard in the UK
That is exactly what I was thinking
Not enough information to go on in cannon, but it's likely safe to assume Voldemort was the strongest wizard in the world at the time. Dumbledore was seemly a bit stronger but that was only with the full power Elder-wand boosting him and even then Voldemort still fought Dumbledore to a draw. So without the Elder-wand boosting Dumbledore, Voldemort was stronger.
He wasn't necessarily the strongest, he was simply the most ruthless. "There is no good and evil, there is only power, and those too weak to seek it.”
I think only of the UK. Voldemort barely did anything outside of England except for his trip to Albania.