Hardest Dungeon?

[Canadane]

A lot of the old ARR dungeons at end game were rough when they first came out. Old Amdepor keep originally had two bug adds with the wall and people frequently fell off the sides when the boss did the knock back.

When they silently removed those adds from Demon Wall, it was a sad day!

[Espon]

I still struggle with Pharos Sirius whenever I get it in Duty Roulette. Those first 2 bosses can be brutal if you don't do the mechanics properly.

[Rilifane]

ARR/ post-ARR:
definitely Pharos Sirius although Keeper of the Lake gave most parties a hard time as well. Especially the last boss... I've seen tons of wipes there.
Honorable mention to Cutter's Cry (mostly last boss), Stone Vigil and Aurum Vale. Seen more wipes in them than in all ShB and EW leveling dungeons combined.

HW/ post HW:
Hands-down "Sickness must be purged"-Vault.
The last boss is still interesting and I tend to see the occasional wipe there but when HW was current, it was on another level. Trash mobs had a habit of chewing on healers because there were often two casters standing too far apart to hit them both with aoes, the giants hit really hard and their conal aoes still one-shot any non-tank. First boss was generally okay but second boss slapped the tank pretty hard and with the longer fight duration from lower potencies, it was a struggle to find a safe spot between the orbs stacking vuln up. A struggle that usually resulted in failure and resetting stacks with Ress.

SB:
Bardam's Mettle, notorious for wiping both inexperienced and experienced parties. Trash hit like trucks on steroids and the last boss had a lot party damage in addition to frequent mechanics. Nowadays you often skip the add phase entirely but the first two wall to walls and the pull with the bird are still spicier than most wall to walls.

Holminster has a spicy wall to wall and the 2nd boss has fairly frequent party damage but I've seen far less wipes there than in Bardam's or Vault. Same for any EW dungeon. They got a lot more creative with mechanics and give people more to do while smashing their buttons but there is always enough breathing room to recover. On Vault endboss it was easy to fall so far behind that it resulted in a wipe. And you better respected mechanics in Pharos Sirius or you were all dead. Not just one person, everyone.

[Hikuro_Kenshi]

Aurum Vale will always be the hardest dungeon in my experience.

It's just the one dungeon that isn't friendly to new players and causes endless frustration to anyone unfortunate enough to be qued into that golden hellscape.