Counting nerfs in Every balance change by hero logged
Mercy (14) > Brigitte (9) > D.Va, Doomfist, Soldier:76, Roadhog, Zarya (6) > Hanzo, Genji, Ana (5) > Junkrat, Widowmaker (4) > Pharah, McCree, Zenyatta, Lúcio (3 nerfs) > rest
Counts above include reverts like the Mercy healing rate revert but not reworks. With nerfs within reworks Symmetra and Hanzo would be shared 3rd place (8) and Torbjörn and D.Va shared 5th place (6 nerfs) as far as I can count.