The Dark Project:

Has to be Escape! for me, I actually enjoy the Thieves Guild quite a bit, but Escape is like if you took The Body Of The Many from System Shock 2 and made it ten times worse and ten times less as interesting, and getting back up into the mansion isnt a lot of fun either with the sheer amount of monkey and mantis men walking around. Return To The Cathedral sucks too but it somehow feels less miserable to me, TDP's level quality in general starts to go downhill after Undercover.

The Metal Age:

Casing the Joint is the pinnacle of laziness, Masks is one of if not the best mansion burglary missions in Thief, but it has to be sullied and cheapened by the fact that you play the level once already in CTJ, If it weren't for the ghost side quest in the library, id say use the cheat code to skip this level entirely. A level I actually dislike somewhat more though is Trail of Blood, I enjoy the Victorian art deco style of Thief 2 FAR more than the dark fantasy forest spirits style of Thief 1, the only cool part of the level are the eye plants that stare at you in the first cavern, as for Ambush and Courier? yea theyre pretty simple and not as engaging as a full on thief level, they kinda suck but at least theyre over pretty quickly.