Wellm this was interesting as it inspired me to try and find my most-read story. Turns out it's this:

https://medium.com/crime-scenes/beheaded-on-a-bus-the-murder-of-tim-mclean-9d5b7ac44efa

Weirdly though, it's not my highest-earning story, because that's this one:

https://medium.com/@v-s-williams/why-meghan-markle-never-stood-a-chance-840ba7d8e7d2

I read that if you just search for your handle on Medium the first result is your "top" story, but that one is neither my most-read or my highest earning, so heck knows what criteria that function uses!