Royal Gel

Equip it and nearly every slime in the world walks straight past. They will not chase, they do no contact damage, and the ones that shoot stop shooting, even after you have hit them first. Your minions and the town NPCs never got the memo and keep killing them, so the drops still come in.

Slime Rain becomes free loot instead of a siege. Building the Wall of Flesh bridge stops being a Lava Slime problem, which matters because of the puddle they leave behind on death. Early trips into the Underground Jungle and the ice caves get much calmer once Spiked Jungle Slimes and Spiked Ice Slimes cannot throw thorns at you. Later it stops Gastropods lasering you across the Hallow while you fish.

Both slime bosses ignore it, along with the spiked and crystal minions they summon. The plain blue slimes King Slime spits out are pacified, which is a small mercy in the middle of that fight.