For the worst ability, it's hands down Haunt. Haunt used to be this ability where your entire team could see an opponent who splatted you for 10ish seconds. In this game, they're marked with the thermal ink effect for 30 seconds. But it's only for your eyes.
Which takes most of the utility Haunt had in the first game. Haunt was able to inform the team about a flanker, or have someone revenge kill the haunted player, because the team could easily see them and punish them.
In this game, the haunt is personal. It's a 1v1 deal. Say your opponent kills you in a trade. And that you killed the opponent after they just killed you. The game will take haunt away from you. There you go, your main shirt ability gone. Say you're close to the opponent, but they're behind cover. You don't get to see them, literally taking the point out of haunt. Say you die, and your opponent dies to your teammate immediately afterward. Nope, no haunting allowed. Again, making the perk worthless.
So I'd definitely say the worst ability in the game is haunt. There isn't much utility behind it, and it's a solo ability for a team based game like splatoon. (Just change it back to what it was in S1 Nintendo)