p
Sigma and Ramattra do a fantastic job at countering Cassidy.
There is nowhere that Cassidy can position that both tanks can't reach, both in terms of shielding and offensive pressure.
Both tanks can both force out and punish an ill-timed Combat Roll. Both tanks are solid in solo queue and in more coordinated environments.
And, both tanks can bully him further with their ultimates. Triple win all around.
Most of the more mobile cast in Overwatch counters Cassidy well
Winston and D.Va both create opportunities for your DPS to pounce on him once his Combat Roll has been used.
This also ignores their own solo pressure, which Winston and D.Va can be a problem if Cassidy doesn't respect you.
However, for both tanks, you're going to need to invest a bit more to protect yourself as you engage to block Magnetic Grenade.
The last thing you want to do is fall short when leaping or boosting in.
If there is more of a flat map and Cassidy is becoming too much of a problem for your DPS to handle on their own, Orisa is your one-stop shop for all things bully. Is she unkillable? Yes.
Does she have displacement? Yes.
Can she just primary fire and walk at him? Yes.
If all else fails you and you just need a target shoved in a corner for the rest of your team to work, Orisa is the counter.