We can see who earned the most reputation in the current week, current month, current quarter, current year or all time. There is this great overview for that. Also see the below screenshot.
What struck me as odd was the different users for the Mathematics site. So I clicked on the picture that is listed as top user for the current year, which led me to this overview. Also see the below screenshot:
Here a different user is shown as the top user for 2019 when compared to the overall overview. I suspect the week or month listing to be somewhat volatile, but the difference between both users in this case is so big I can hardly blame caching.