having seen a fair bit of both, i'd say that One Punch Man and Hero Academia are hard to compare but are about as good as each other. OPM had a bit more punch (no Pun intended) and was flashier from the get go. it was more of a comedy. MHA burns slower and has more of a heartfelt narrative, but because it's longer with a larger cast, it can tell better individual stories than OPM could and has an overarching narrative that is a bit more ambitious. OPM dazzles. MHA captivates.
MHA is like that episode where Mumen Rider takes on the Ocean King, but as a series.