Hidden Agenda completely blew me out of the water. It's a typical Mechanist mission, akin to the missions "Eavesdropping" or the Angelwatch section in "Life of the Party". The execution of the setting is where this mission shines. For starters, it's huge, and almost wholly dedicated to interior spaces that form part of a big Mechanist complex. This makes for interconnected level design, and lots of different rooms to explore. Secondly, the complex, and the way you open it up, is laid out in an excellent manner. Unraveling the Newmarket Mechanist Seminary takes time. A lot of doors cannot just be lockpicked. You have to find keys. This requires a thorough combing through the richly detailed parts that make up the location. A story is slowly revealed, and the structure this mission has allows it to be unveiled in a pace that even official missions fail to match. The story isn't groundbreaking, but it's effective (and frankly, I didn't expect a real story to begin with, let alone some very high quality custom voice lines).

The atmosphere makes for a mission that sometimes feels intriguing, at other times creepy, and occasionally even cozy. Most of it feels like the standard Mechanist atmosphere, but certain rooms just feel like they have more of a sense of placement, a sense of space in the building. They contain more of an in-universe purpose. Once or twice, there's even a Dark Souls moment of "wow, I never expected this path to lead me back here, but from an entirely different angle." It's amazing. Also, shout-out to the map. Every small room has its own representation on the map, rather than being part of a bigger block (looking at you, T2X). This made it really easy to see which rooms I had missed, no matter how insignificant they looked on the map.

All in all, this might be my favorite mission in Thief as of yet. It's criminal to see this mission hasn't gotten any reviews yet. I hope mine will be the first of many.

SPOILERS:
I did enjoy the underground portion a little less than the parts on the surface. I'm glad the objective of finding the hammer was optional, because the Haunt section felt very restrictive. Invincible Haunts don't really make my day. It's why I gave the gameplay a 9 instead of a 10.

Heist Society and Night of Trickster are also good.