Sigma can control the fundamental forces of the universe.
Symmetra can create literally anything out of hard-light.
Doomfist has a gauntlet that can supposedly level a skyscraper.
Both Reaper and Moira are essentially immortal.
Sombra is the most dangerous hacker in the world.
Tracer can control her own time.
What do you think?
My Personal Tier List (in order)
S: Sigma
A: Doomfist, Sombra, Symmetra
B: Moira, Reaper
C: Zarya, Tracer, Mercy(?)
D: Everyone else.
In current animated shorts/origin stories/comics probably Doom.
He stood toe-to-toe in a 1v3 with Genji/Tracer/Winston in his origin story without breaking a sweat
In future story cutscenes or shorts, probably Sigma if Talon uses him for their purposes. Right now he’s not being used for anything
Sigma. Doomfist and Symmetra both get shut down by Sombra. Anyone who requires technology basically does, so Tracer is out. And if not Sigma, because he can’t control himself, Winston.
Doom can ‘level a skyscraper’
Like, power level wise? Well, technically it’d either be Sigma or Zarya, since they both mess with gravity tech. Of course, an argument could be made for Moira, who has somehow invented tech that can sap someone of their life force and transmute into her own life force.
But power levels are BS anyway, especially in regards to villains. We all know they’re going to lose anyway, especially in a story like Overwatch that’s isn’t dark enough to have the villains win, regardless of logic.
Sigma or Symmetra. Just because Sombra can hack Symmetra, it doesn’t make her overall less powerful compared to the rest. Also Sombra can hack Signa ingame…mh.
I still think some heroes would have soloed the big omnic in the ow2 trailer.
This did not help him against Winston.
Genji:
Current Genji yes 
Hog.
Those legs need to move that bacon around.
I’ve seen this discussion before and the conclusion is Doom.
Mercy… she can bring back people from the dead.
I think a big component in ranking them would be based on if they can use the tech they invented/utilize or not.
Is Zarya’s graviton even canon?
Doomfist only lost because the plot demanded it.
Pretty sure that’s not canon
Symm and sombra. No competition.
Yeah but its not like there is much plot about these 2… We have to go by feats instead of the statement of a 9year old child.
Say bacon. One. More. Time 
What is a Roadhog on fire called?
beacon
reported directly to Jeff
Stop it! you would make him hungry…
Sym, Symmetra could create anything and if anyone had more powerful equipment than her, she could just recreate it for herself.