Zarya

The high damage of primary fire while charged.

With a fully charged primary fire, Zarya can deal 175 dps. This is almost the same DPS as Soldier: 76's primary fire, so when fully charged, and considering her range, she puts out top-level damage among tanks.

Projected Barrier on allied flankers

By using Projected Barrier on allied flankers who deal high damage at close range, such as Vendetta, Genji, or Reaper, you can create a temporarily invincible flanker, applying heavy pressure on the enemy backline. Also, when Zarya herself uses Particle Barrier she is easily watched for, but a flanker is less likely to be watched, making it easier to build charge with Projected Barrier.

Particle Barrier can cleanse debuffs.

By using Particle Barrier reactively, Zarya can cancel most debuffs, such as Nade, burn effects, and hacks.

Count the Particle Barrier cooldown.

Zarya's Particle Barrier stacks up to 2 charges, and the cooldown is a long 9 seconds. So after the enemy Zarya uses both barriers back-to-back, a big opening is created, which is a chance to gain an advantage.

Melt her down together with the barrier using a hero with high DPS.

If Zarya's charge builds up no matter what, one option is to melt her down all at once with a high-DPS hero such as Bastion or Mauga.

Attack with feints.

When Zarya has Particle Barrier available, feinting by splitting up your primary fire into small bursts can make her waste Particle Barrier. You can also deliberately use an ability with a wind-up and aim it at someone other than Zarya to bait it out. For example, with Rock, aim at Zarya while holding the rock, then at the moment you throw it, shift your aim onto an enemy other than Zarya to fire it, making it easy to bait out Particle Barrier.

Zarya's barrier has an 11-second cooldown when used on herself, but a 9-second cooldown when used on an ally, so using it on allies is more cooldown-efficient.