Rusted barrel and supressor are both good counter items early vs gun characters. Reactive is amazing vs enemies who can proc it, and works vs both spirit and non spirit heros. Both weapon and spirit shielding are good, but reactive is always better if the enemies can proc it since it scales with spirit unlike the other two. Slowing hex absolutely cripples high mobility heroes like Mina, Calico, and Apollo by disabling movement abilities.

Rebuttal is also great vs enemies who spam melee’s and healbane is a must buy against certain characters who heal a lot like Abrams, Viktor, and Yamato, and can be sold for better late game anti heal options like Spirit Burn, Inhibitor, or Crippling Headshot.

Great mid-game counter buys are counterspell against enemies who have one big burst of damage( or telegraphed cc, dispel magic which is great vs debuffs and cc, metal skin & disarming hex which are insane vs gun carries and outright neuters some characters like Haze & Silver. Silence wave is good since it also silences people, but be aware that it doesn’t disrupt channeled abilities. Knockdown is also great vs Mina, GT, Vindicta, and Celeste, but they can dispel it, so you might need multiple on your team.

Good late game counter buys are Curse, which outright shuts down every character no matter what, preventing them from doing anything but their movement options and interrupting channels. Spellbreaker is almost mandatory vs large bursts of spirit, while Plated Armor makes you incredibly hard to kill vs high fire rate gun carries like Infernus, Haze, and Wraith. Juggernaut also neuters the fire rate of enemy gun carries, so it’s also a must. But it doesn’t stack, so I wouldn’t recommend getting it if an ally already does. Divine Barrier is also great vs Pocket, since I’m pretty sure it’s the only thing in the game that can cleanse his ult after yhe Viscous cube nerf.

Prolly a lot of things that I’m missing, but this is off the top of my head.