Some recommendations:
Ominous Bequest and its 2 mission sequel Broken Triad. They are all very varied and interesting, and keep giving you new mission goals as you play. Ominous Bequest starts as a pretty normal mansion mission, then you uncover some secrets in the catacombs in the basement... very varied mission, judged by some people to be the best Thief FM ever made. The sequel was equally well recieved, with its first mission being a relatively large city mission with a museum to rob and mysterious things happening in the city. Streets to explore, multiple buildings to break in, and an eerie atmosphere at some places (and it has a really awesome optional area with a cool reward at the end). Second mission has you visit a place similar to the lost city, it's a bit heavy on puzzles but it's pretty fun and has a great visual style and some unique enemies.
The Bathory Campaign. This is one of my favourites. It's a campaign with several missions in a historical setting that just oozes with atmosphere. I really liked the design of the castles you visit in this one, and the story is pretty interesting too.
Tales from the Cleft: Vandal and Tales from the Cleft: Ladytaker. First one is a very solid mansion mission, second is a very solid city mission with some cool secrets to discover.
Mystic Gems I and II (Unlucky Soul and Heart of Bohn): first one is a smallish mansion mission, second has you rob a large museum. I enjoyed them both greatly.
Heist Society. A very good mansion mission, it's quite an immense mansion you're breaking into there (been a long time since I played it, but I'd say it's comparable in size to First City Bank and Trust).
Wicked Relics. Probably my favourite FM. It has some interesting hidden secrets, a humorous story, and a really really big mansion with a lot of loot to steal and a lot to discover, including a moment of shit hitting the fan and strong undead being spawned when you complete a certain objective.
The Widow's Ire. A mission that appeared in 2013 by a new author and surprised everyone in how fucking awesome and weird it was. It's a two mission pack, with the first being like a mix of The Sword's garden area and the Bonehoard, and the second being very much like The Sword, only different - it has a big mansion with an even bigger WTF-area coming after it. If there's any FM that manages to capture the weirdness of Thief 1's more supernatural missions, it's this.
Play these, they're some of the best in my opinion, and after that you should try some of Christine's missions (especially her later ones, Coterie of Smokers is one of her best),
skacky's missions and
Melan's missions (they both love doing city missions with lots of verticality).
Have fun taffing!