Bloodstone prison is a classic. It is one of those maps that has aged amazingly well, and holds up as one of the strongest FMs of it's generation.
Honestly, my only issues with this FM stem from it being a bit smarter than I expected: one of the locks is actually a puzzle, and one which while easy enough to solve, I simply wasn't looking for a puzzle solution.

This is an absolute gem of a mission, holds up well, has good atmosphere, and can be mostly completed stealthy. It's just a good time all around. If you like undead, and like ruined hammerite buildings, this here is better than most official thief missions.