The mobs you hated to face

For CoT.... Spectral Followers, Demons, knights. Then the Spectral Lords (the ghost lts) That -acc and the level range that appear is a pita.

Vahzilok: The Mire and Murk Eidolon.

Vanguard: just.. wow.

Also not fond of fighting arachnos.

Snipers.. I have a special hate for snipers regardless of group.

Anything with excessive endurance drain or -recovery. Mainly: Carnies and Arachnos

Definitely Master Illusionists. They just slow everything down with all those pets and their phase shifting. It gets a little difficult to even see if they're tangible or not with all their glowing summoned jerks floating about everywhere

(54x8) Carnies. But I also love fighting them because of how much I hate them. Every time I'm like, "I'm gonna best these cheating bastards." And I've pulled it off a few times too.

Oh and I forgot about Arachnos. I just hate them because they're evil. Stacking -end from multiple sources, so I can't just kill the Sapper like with Malta. And on top of that, piling on -defense. I an an endless, defenseless level 12 pre-i22 Stalker at all times. Cringe. I've never ever solo cleared a full map of 54x8 Arachnos on any character. They're too evil.

I hated the Dark Astoria faction with the Sybils and Harridans, whose nameforget.
That knock-up sand blast attack with the massive graphics that obscured the entire screen at the default camera settings was just obnoxious.

They weren't especially hard to fight, just not much fun due to the constant camera jerking and obscured vision.

Towards the end of days, there weren't any I hated on 50s, because my 50s were all swimming in IOs and Incarnate powers. Those I found challenging and/or had to be cautious about were, in roughly descending order...

• Arachnos - These guys were, IMO, the premier scary dudes in the game outside of Incarnate foes in DA. Loads of exotic damage - toxic, psi and energy. Loads of energy drain in the Mu critters. Loads of slows in the various web grenades, some of the spider venom and lots of the Psi attacks. Plenty of mez, though not overwhelming amounts like the Malta. Night Widows could blind you and were crazy hard to hit. But most dangerous of all for multiplying the danger of all of the above were the Tarantula Mistresses, with their autohit, -30% defense attack that also blinded you. Holy crap, did that power suck.
• IDF - These guys were sort of "Arachnos-light", but mostly not as bad. They also had an auto-hit toHit/DR debuff, but it was delivered by minions, which made it a bit easier to counter early if you saw them. Wicked end/recovery drain from the Seers.
• Malta - Not because of Sappers. Sappers sucked, but I could usually mitigate them because there was only one per spawn. No, Malta sucked because all the human troops but Gunslingers could debuff your defense. They were like Cimerorans, but at range. It always surprised me how few people talked about this. Also, upgrades to the powers and AI of the Titans and Gunslingers made them extremely dangerous. Two Gunslingers, two Titans or a mix of them could mez my Scrappers and Brutes, making them deeply problematic for many squishies. And I'm not even talking about the 30s-long stun grenades - don't get me started on those.
• Longbow - Longbow were, mostly, two-trick ponies. They were another group that debuffed the living hell out of your defense at range, and then debuffed the hell out of your DR. There was little you could do about this in a large spawn, because every minion and LT basically had the same powers - the annoyance they offered was totally distributed through a large spawn. The bosses were varied and powerful, and magnified in threat by the debuffs applied by the minions and LTs.
• Incarnate Talons of Vengeance - Holy stacked debuffs, Batman. If I were to have ever fought these guys at +2 or +3, I would probably rank them my scariest opponents, but I only ever fought them with +3 Incarnates. Despite incarnate powers, I found myself able to suffer sudden, unexpected defeat at the hands of these foes more often than anything except Arachnos and Longbow. The really scary thing is that Arachnos and Longbow usually achieved that through cascade defense failure that I failed to either notice in time or react to in time. ToV would often kill me through stacked effects that weren't debuffs, like Flow Lightning. Of course they actually did have some wicked debuffs, mostly in the form of Sirens. The challenge difference after Sirens were added was really dramatic.
• Incarnate Banished Pantheon - If I were to have ever fought these guys at +2 or +3, I would probably rank them higher, but I only ever fought them with +3 Incarnates. They are mostly only dangerous because of the -def stacking through Elders and Ancients of Sorrow, though they have some other ugly debuffs, particularly -DR and -recovery.
• PPD Quantums. OMGWTFDefenseDebuffs - IMO, nothing had it on Quantum PPD for defense debuffing, not even Cimerorans. Every attack debuffed defense while delivering energy damage, and they could do it at range. Oh, and bosses and LTs could res, making it harder to win through overloading on buffs through insps or whatnot.
• PPD SWAT (Honorable mention, non-50s) - I found these guys were the most annoying foes in the game, hands down. They could slow your movement as badly as the worst stacked caltrops, but it was an applied effect, not just a patch. They could blind you. They could debuff defense at range. They could apply massive toHit debuffs at range, in an AoE.
  
  ...except you met PPD SWAT as a villain at level 20. I found them annoying on 50s playing on high difficulties. On 20-somethings they were overwhelmingly crippling at x1. I found myself standing around sometimes minutes at a time between spawns waiting for their horrific debuffs to wear off. That was completely uncalled for.
• Cimerorans - I wasn't really ever afraid of these guys, but they do deserve mention. They debuffed defense like pros, were immune to lots of mez when close together (at least once the were aggroed) and they hit like trucks. However, they didn't really rate with the foes above because they dealt pure L/S damage, were gimp at range, and had no special protection from fear or confuse, which lots of my squishy characters were good at.

OMG before reading Ubers post I had almost forgotten about the SWAT PITAs. This was one of those situations where I didn't even care if there was a BOSS in the mob I was heading straight for that stupid Equalizer to TRY and prevent that gooey mess that slowed me to a crawl. While it wasn't quite as big a deal for my ranged characters that stuff gave my melee characters fits. And as Uber wrote the effect was long lasting so even after yu cleared a mob it was a while before you were moving normally again .. and then you'd get slimed again by the next mob .. Grrrrrr