Brigitte. Don’t be dumb: Tracer wins every 1v1 and that fact doesn’t have anything to do with counters. Brigitte makes killing people insanely hard for Tracer. In a coordinated team she makes Tracer not matter. Brigitte herself has 500 hp against Tracer, she always lasts long enough for teammates to peel and is this a crummy target.
Sigma also wrecks Tracer. Not needing to aim and hitting around corners is just back breaking for her, and he can just dump out a barrier and take forever to kill.