Symmetra

Area defense capability via Turret.

By sticking Turret in enemy blind spots such as corridor exits or above gates, it becomes much easier to defend that area. It's easy to leverage on defense in Control, Flashpoint, or Payload.

Free-flowing playstyle via Teleporter.

Teleporter can be used in many ways. By putting an ally through it she can apply huge pressure on the enemy backline, or by sending herself back and forth she can play like a flanker. She can freely change her strategy depending on the enemy.

High DPS from primary fire.

Symmetra's primary fire has very high DPS once charged, so she can build charge on enemy tanks or enemy barriers and then melt tanks with that high firepower, or push her high-charged primary fire onto the enemy backline.

Don't carelessly put up barriers.

Symmetra's primary fire charge can also be built up on barriers, so she can charge on fixed barriers from Ramattra, Sigma, or Domina and then attack all at once. Tanks need to be careful not to let this happen.

Prioritize targeting Symmetra.

When Symmetra is using her primary fire, you need to prioritize targeting her to prevent her from building charge or to apply kill pressure. Left unchecked, Symmetra's primary fire charge builds up before you know it and becomes unmanageable.

Fight with a composition of high-poke heroes.

Symmetra's poke power isn't that high, so poking her for damage in advance makes it hard for her to push forward.

Symmetra's teleporter can also warp objects like D.Va's mech during self-destruct or Torbjörn's turret.