(PG, Jul 2021): Submissions with high readership over the past decade or so, all above some tens of thousands of readers cutoff (earlier readership data exists, but hasn't been processed). Sometimes the readership correlates with importance, sometimes it's for spurious reasons, sometimes for no obvious reason.
And some important papers do not appear. But a quick scan might help surface important articles otherwise overlooked

astro-ph
cond-mat
cs
econ
eess
gr-qc
hep-ex
hep-lat
hep-ph
hep-th
math
math-ph
nlin
nucl-ex
physics
q-bio
q-fin
quant-ph
stat