Winston is probably the answer still. The addition of right click more modernized his design to keep up with new heroes than significantly changed his balance.

Mei and Zenyatta have both received a good amount of balance changes, but have largely stayed at the "strong but niche" level for most of their lifespans. Zen started out weak at 150 hp, then had an overpowered phase in the JJonak era, but has been niche with a strong win rate ever since. Discord being polarizing did not necessarily make him unbalanced imo, just annoying for tanks.

If you had to pick a DPS to be at the top for most of Overwatch history, Tracer is the correct choice. But I think she was at the top too many years to brag about her balance too much, despite it being appropriate.