The most important one by far is the heavy melee sound. Spend some time in the sandbox with your back turned to the parry practice dummy and try to learn to instinctively press parry when you hear the sound. I'm still not great at it but it's won me some lanes when the enemy is meleeing creeps.
I’ve only got it once so far, but diving into the hitbox of an Abrams heavy melee to parry and protect an ally is THE MOST satisfying experience in any game.