-WORST OF THE WORST-
1. Those bats in Zelda II that like to hang around instant death pits. SO. MUCH. RAGE.
2. Bubbles, Like Likes and Blue Darknuts/Wizzrobes from the original Zelda. On their own, they're a pain in the butt. In one room....-shudder-
3. Beamos. Specifically, the Beamos in any of the games where they're invincible, and the three Beamos guarding the boss key in the pirate ship dungeon in Skyward Sword.
Honorable Mentions:
-Ice Keese. They're irritating, but they're relatively rare, easily dealt with, and are nowhere near as annoying as the above mentioned enemies.
-The Armos in Skyward Sword. Ugh. Pain in the rear.