Here is a list of current Symmetra counters:
Zenyatta - 30% discord evens the playing field against shield generator.
D.Va - Eats up orb spam, which significantly hinders Sym's ability to build ult. She is also the best hero to use to destroy shield generator. She is pretty necessary against a Sym now.
Tracer - Blink past her setup and eliminate her before she can get the generator up. Sym should have trouble 1v1ing you if you catch her out of position or the rest of her team doesn't give a fuck about her.
Winston - Good against sentry nests.
Roadhog/Widow/Hanzo - Get an early pick onto the Symmetra to stop her from getting shield generator down.
More indepth: I feel like Zen and D.Va are your best bets at a fairly decent SR, diamond and above. Roadhog is meta right now, so running him isn't much of a gamble with how much else he can accomplish. Tracer and snipers will be much harder to accomplish at higher levels, especially snipers. That would require the Sym to be bad and not respect the hook and Widow/Hanzo sightlines, or a charge-happy Rein who leaves her open to be picked off.
If you can dive a Symmetra, she is not going to last. You just got to plan and figure out how to accomplish this. If she is well protected, you gotta play the shield game. Which Sombra excels at, and she gets rid of her additional shields for 6 seconds with EMP.
I'd add Pharah, her rockets destroy sentries as well and any half decent Pharah isn't gonna be hit by a Symmetra's gun. Like. Ever.
Pharah would surely work against a Sym+Torb comp, which usually isn't running a Soldier or McCree. Against Sym alone, it just depends if they have the hitscan, which is just basically the enabler or disabler for a Pharah. Going back to Sym+Torb vs. Pharah on a map like Gibraltar, holding in the server room will make it hard for Pharah to actually do anything in that situation. Seems like a very tough defense to crack, regardless of whether NRG wasn't playing well or not.
