As the title says, what are your personal underrated Dark Mod (and Thief fan missions in general if you want) missions? Missions that are good/great/fun that never got a lot of attention or have otherwise faded away, in essence.
Personally, I'll give a nod to Briarwood Cathedral by buck28. Not the best mission, and some of your goals can be obtuse, but it somehow hit this vein of pure *fun* that I can't articulate well. The same with his first mission, Lockton Manor.
I'll also give a nod to Lords and Legacy by Kvorning. A fun, complex city/mansion mission that the author dropped on us all and then vanished without a trace. It's very fun and I'm tempted to give it another go one of these days, and it's a shame that I don't see it mentioned more often.