Arreat Crater Level 2, Soul Lashers with teleport / fast / shielding / reflect damage.
What the ... I've killed 2 after dieing 3 times. It's the only mob I've considered running by, but I can't. They catch up in an instant.
I'm able to tank 1 minion/elite hit with 49k HP, 350-400res, 3200 armor, 60k dmg. And with gloom maybe 3/4 it's insane. Smoke screen build didn't really help... man elite Soul Lasher's were already a pain.. but with these affixes.. damn.
So what is your worst nightmare? Depends alot on playstyle of course.
Hmm. I think that's about the worst for me, too. Invulnerable Minions, Horde, Reflect Damage, and either Fast or Teleport might tie or come in at a close second.
I did run into, of all things, a Fallen Shaman + Fallen pack with Invulnerable Minions and Fast; it was pretty nasty. I couldn't keep the fallen off of me very long and any time I damaged the Shaman he would run away (quickly, being Fast). Died too often to put them down; I eventually parked that pack out of the way.
Shielding, Hoard, Frozen, Extra Health Bees in act 2. Wiped for 45 minutes before I brought them down. At that point it was principle. But soul lashers with anything are up there. I run Screaming Skull instead of NT because the fear is the only thing that can save me against them.
I ran into a snake demon with Shielding. That was the worst ever. The first elite I've run into where I just gave up. I couldn't kill him. Couldn't even come close. When he wasn't shielded, he was invisible. As soon as he dropped invisibility, the shielding would come up. He was vulnerable to damage MAYBE 5% of the time, and was healing far faster than I could hurt him.