OW Hero: Reaper

Soldier76 cannot survive in a 1 vs. 1 fight against Reaper. He has lower health, lower damage capabilities, no escape option and his healing does not nearly heal him quickly enough to make a difference in the fight.

As S76, keep an eye out on where the opponent's Reaper is and try to make sure its not heading toward you.

It somewhat depends on the situation.
At medium range Reaper stands no chance, a good Soldier: 76 player can most likely land the Helix Rockets and get a good few rounds around his face and upper torso, killing him before he can properly react. However once this gap is closed it would take a miracle for Soldier: 76 to come out on top, considering the high damage output of Reapers shotguns and his ability to change into Wraith Form when things aren't going his way.
But since it mainly depends on the range of engagement, and since reaper has a lot of abilities to avoid direct fire and close the gap, it's gotta go to reaper.