Nit: if it were top mathematicians of their age, then there wouldn’t be any ages without first-rate mathematicians, which is obviously wrong.
That said, the “of all time” version also doesn’t sound right.
That said, the “of all time” version also doesn’t sound right.
"Who are the first-rate/first rank musicians of $YEAR" does not seem to me like it could be answered with "there were none in that year".