I’m just fighting Kurzen Medicine Men in STV and I can’t believe how annoying they are. They heal to full hp from ~20%, and their renew almost outheals my wand damage.
Not to mention they run away in fear and often respawn very quickly.
Any other NPCs you’ve found particularly annoying on hardcore?
Hmm. Any mob that heals itself is a pain but the worst ones are thrash mobs.
Some of those raptors in the Arathi Highlands have nearly sent me to an early grave after hitting me 3 times in a row and 2 being crits 
Not specially on HC, But Fandral Staghelm always annoy me. His dialogue is so … Unpleasant.
Spoiler - he is a traitor
Dont know the name of the mob, but in darkshore top right inside the tower… the warlocks in there have extremely high hp and literally do not oom. Their pet also has extremely high hp.
Dont go in there… alone
The inkspewers in Swamp of Sorrows with their constant miss-chance debuffs. Ewww.
The tower of Athalaxx. The very place my first WoW character a lvl 16 Night Elf warrior Karnagen learned in May 2005 that skull instead of level doesn’t mean that it is a boss, but in case with common mob it is 10+ levels above you.
Shan’do Staghelm did nothing wrong. He’s one of the most interesting characters in lore. He acted very reasonably in every circumstance he was in, except he didn’t have the insane plot armor of the 3 main kaldorei heroes. Then the dragonflights decided to screw him over twice for literally no discernible reason. Lol no we won’t help fight the old god bug army and wait until your son gets killd in front of you and the bugs take over third of Kalimdor, then, when our Caverns are threatened we’ll just swoop in, strafe them a couple of times and seal the bugs in. Here, have a scepter of the shifting sands to remind you of how we stood by for your son to get killed. What, you want to regrow the world tree because the last one was blown up when we didn’t lift a finger to fight a world endig demonic invasion? Lol no we won’t bless a new tree, because reasons. No wonder he went over to old gods in the end, if your side keeps screwing everything up for you.
Not to mention how Malfurion blew up the tree to begin with, and then just went straight back to hibernating. I guess you’re the Archdruid now Fandral, go pick up a pieces of newly mortal kaldorei civilization while I take a nap lol. Btw the bugs that are your lifelong nemesis are waking up. GLHF
Edit: oh as for the topic, I really dislike mobs that either lock you out of a spell school with a kick or that are flat out immune. Water elementals in Ashenvale and Feralas come to mind.
At low level it has to be the Nighsaber kittens that hang around with their mother. You think I have it and 3 of those turn up. Like that mother bear in Darkshore.
Oh and anything with a really fast spawn rate.
Syndicate assassins in Southshore. One just gave me a heartattack when it jumped my 25 druid as I was walking into town. Luckily a nearby guard got it, before it got me.
He also gave druids the firecat model, which is still one of the best druid forms in the modern game.
…It’s funny how you can basically justify a lot of WoW villains by pointing out just how stupid the ‘heroes’ of the story were. Even the black dragonflight do that in the northrend quest, when they point out that Alexstrazsa asks for aid with the ruby dragonshrie but makes no effort to help anybody else with theirs.