There aren't too many rankings of FMs out there, so here's my personal ranking list. A 10 would be the best level I have ever played (in all thief games), while a 1 would be the worst. This list should be spoiler free.

/10	Src	Mission	Notes
10	FM	CoSaS2: Mission X	I have no idea how anyone is ever going to top this.
10	FM	Calendra's Legacy	I actually felt like I was in a 'living', breathing city.
10	T1	The Sword	The gold standard of level design, storytelling, and atmosphere
9	FM	CoSaS1: Gathering At the Inn	Provides many options, with every option seeming grounded/real
9	T2	Shipping and Receiving	This level exposes so much about the world, while being super fun.
9	T2	Blackmail	A classic, which plays great all the way through!
9	T2	Life of the Party	Somehow this level feels larger than all the others combined.
9	T2	Casing the Joint	A mansion specifically built with secrets to find!
9	TBP	The Long Shadow Falls	An excellent heist mission, if only there were more secrets.
9	2x	Unexpected Shelter	Super linear, but a fantastic intro.
9	FM	Calendra's Cistern	A fantasic, fleshed out city with tons of secrets and paths.
9	TBP	Kept away from view	Lots of little details and quite an exciting premise!
8	FM	The Tower	Shockingly small level for the content.
8	T1	Assasins	The first mission where I realized I loved this game.
8	T1	Break From Cragscleft Prison
8	T1	The Lost City	The map in this mission is a work of art.
8	T2	Framed	Starts off very linear, but explodes after that.
8	T2	First City Bank And Trust	The bank itself is great, but the basement is rather weak.
8	T2	Precious Cargo	The twist in this level is great, and there's lots to explore here.
8	TBP	Jaws of Darkness	Fantastically huge with tons of detail, but much too linear for me.
8	2x	The Trials That Shape Us	So many secrets to find in an unassuming setting!
8	2x	The Redistribution Game	I like the mix between buildings and caves here.
8	GB	Slipping Away	Very solid level, unfortunately lacking use of the rope arrow.
8	GB	The Final Thread	A great level, with tons of secrets. Great callbacks as well!
8	TBP	The Brand	Every area kept expanding out with more and more content
8	TBP	Death's Dominion	A much larger and more intricate undead level than I expected
7	T1	The Haunted Cathedral	I like how the map isn't completely accurate here!
7	TG	Song of the Caverns	A weak start leads in to an unexpected and cool finish.
7	T1	Undercover	TG shows its age most in this level, but still a blast.
7	T1	Strange Bedfellows	Great moment story wise.
7	T1	Lord Bafford's Manor	Seemed too normal/standard to resonate with me.
7	T2	Running Interference	Starting T2 off with a bang.
7	T2	Eavesdropping	Great progression of objectives here.
7	2x	Of Ill Repute	Starts off pretty standard, but the inn is fun to explore.
7	2x	The Grand Hotel
7	GB	Impious Pilgrimage	Really cool innovation, tempered with horrible caves and maps.
7	TBP	Where Old Faces Fade	Reminds me a lot of "The sword" - but does not do enough with it.
7	TBP	Trial of Iron	Plenty of secret passages and rope-arrow use balances the plainness.
7	TBP	Arcane Sanctum	Really great opening with some nice moments, but frustrating overall
6	FM	Rowena's Curse	Story is fantastic, but the level has a critical path.
6	T1	Down in the Bonehoard	Parts of this mission are fantastic, but the mazes are painful.
6	TG	The Mage Towers	The first half is great, but later on quickly becomes a grind.
6	T1	Escape	The premise here is very cool, but gameplay is weak.
6	T2	Kidnap	The callback is very cool, but running in caves isn't great.
6	T2	Sabotage at Soulforge	Lots of interesting mechanics, but pretty sad for a last level.
6	2x	Shadowing the Enemy	Notable for the 3 stages of the mission, but otherwise bland.
6	2x	Betrayal	The start of this mission was amazing. Overall a serviceable end.
6	TBP	Return to the City	Lots of detail and verticality, but a lot of dead ends too.
5	FM	The 7 Sisters
5	T2	Trail of Blood	Seemed too linear and uninteresting for my taste.
5	2x	The Art of Deception	Maybe the nicest city map I've played…
4	T2	Trace The Courier	Has nice spots, but overall not fun
4	2x	While the City Sleeps	I don't understand why everyone keeps making these street maps.
4	2x	A Question of Knowledge	This mission got very repetitive, and secrets weren't common.
4	2x	The Cure	Such a large level, but dealing with this many undead isn't fun.
3	T1	Into the Maw of Chaos	Story wise, very exciting, but the level on its own is weak.
3	2x	Down Among Dead Men	Not a fan of this undead mission…
2	TG	Thieves' Guild	Lots of long, long tunnels.
2	T2	Masks	This would be a great level, if it wasn't a duplicate.
2	2x	Into the Fray	There are changes, but too similar for me.
1	T1	Return to the Cathedral	The hidden objectives here were more painful than fun.