Hew's Bane, specifically Abah's Landing.
I despise the multi level layout. You go to where your quest marker is on the map only to find it is 3 stories above you and you have to back track half the map to find the way up and then complete a maze of planks and ledges.
I actually rage quit the Thieves Guild quest line because of it and didn't go back for over a year to do the thieves guild quest line during the event.
Horrible zone design.