I am surprised that Debian has overtaken Ubuntu ! Ubuntu has always been next to Mint in popularity. Must be because of its Unity interface ?
I didn't report the 1 month stats as that may be skewed by a distro with a new / updated release (OpenSUSE a case in point). The 3, 6, and 12 month stats probably show a truer picture of ongoing distro popularity (at least to show what people are viewing, but not necessarily installing).
I also look at that list from time to time and often find the results surprising. I didn't expect to see Kali and Arch so highly placed and yet Qubes is nowhere to be seen. There is at least one forum which only allows starting a sub-forum if the distro in question has been in the top ten of that Distrowatch list at some time .... it always struck me as a little unfair. I can sort of see the reasoning , but would it really cause any extra resource use , clutter or admin work ? Another odd forum rule I have seen is only permitting a new sub-forum if a member of the Dev team is involved.
Think I read Linux Mint is Ubuntu, so if you don't want Ubuntu, Debian is next in popularity. And if you do want Ubuntu, Linux Mint is the logical 1st choice.