Nightfang Spire, all the way.
I've just finished running my group through (a moderately altered) Nightfang Spire. It certainly lived up to its reputation as a meat-grinder, but my players really got into this adventure. They worked together better than they ever have before, simply to survive. We all came away from it very, very satisfied.
To keep up the level of horror-suspense, I filled many empty rooms with random apparitions of former cultists, victims, and adventurers who perished within the Spire. The more my group could learn about the history of the place, the more they despised the villains waiting at the end of it.
In a party of 6 characters, 4 of them died (one even died twice...). But surprisingly, the deaths were the result of combat going against them, not from the Spire's (many) horrific traps.
-eric