Destroyers

If you going to use them to fight cruisers and battleships then yes they would be the worst naval units

Even when you don't, they only have a single purpose... reveal and kill subs. However at the "fight" part of that statement, they aren't really good, and they only barely win a 1v1 fight. So yeah I would agree that in the grand scheme of rock-paper-scissors, they are probably the worst naval unit.

they have some additiona usages: Since they are the fastest naval unit, you can also use them to:

1) quickly scout out sea routes

2) hunt down convoys (they arent as effective in fighting them as other units, but they can intercept them quicker)

3) lock down enemy fleeing ships. Even if the destroyers will be killed in battle, they can lock down the enemy until your real fleet can catch up.