When I was playing rogue, https://classic.wowhead.com/spell=1130/hunters-mark drove me half way insane. An instant cast with a comically long range that kept stealth from actually making me invisible, and that I couldn't remove from myself (even using Vanish) -- it was the worst.
As a mage I hated beyond the point of totall madness: lock felpuppies silencing me and munching away on my buffs
Will of the forsaken. Completely broken racial that made alliance warlocks suffer in pvp, and made everyone and their mothers roll a stinking undead corpse.
For some reason I dislike fighting against (absorb) shields like PW:S, Frost Barrier and Warlock Voidwalker Sacrifice shield.
It feels like I'm following facts in order to chew through the shield or something.
Paladin bubble is my hell.
The first name I will try to catch in-game will be Bob. If Bob is unavailable, I'll become Saltybob. Tremble before me.
I was pretty bad at PvP but playing Alliance mage I really hated fighting shamans. I didn't know their spells well but it seemed like they always had something up their sleeve. Maybe that's why I rolled one in TBC..