I've decided what I dread the most:
Planes. Don't care which type. Just planes in general.
Against destroyers, I can attack sneakily or only in favourable conditions. I can also fight back against them if need be.
Planes.... not so much. You can't really fight back against them, and if you use time compression, you will lose more boats to planes than anything else, as you get bombed and sunk before time compression drops and you get to yell "wtf?"