Awakener on Demon Soul says Shattered Halls is definitely the worst instance of the expansion. I have to completely disagree-- I think Shattered Halls is fun and simple once you've got it down, and the Gauntlet of Flame is one of the best 5man experiences in the game. Then again, not all long experiences are fun-- my pick for worst instance is The Black Morass in the Caverns of Time. Escape from Durnholde was brilliant fun, but BM is just (literally) a swamp-- there's no variety in the spawns, and no story to the event. Either you kill the nondescript bad guys or you don't, and I think it's a waste to be "fighting alongside" Medivh when really he's just standing there yelling at you.
But that's just me. What's the worst instance you've seen in the expansion?