I absolutely hate the Darknuts from TP.
When there were more than one of them, it became so difficult for me to kill them.
In the Cave of Ordeals I was sitting there for about 45 minutes and I was so close to screaming and just throwing the controller out of the window
Apart from that, the Wallmasters. They seemed so creepy when I was a child, I can't help but hate them now more than anything else in those games.