For me, despite having started and enjoying some of my own, my favorite is the one I'm in run by Sweetp4u and ARaynes. Called "Whisper Manor" it is a very horrific visit to a haunted manor by heirs to that manor and the fortune involved. These two ladies are sick... but then, so am I. (joking)
For me, even though I am involved in the thread, my favorite reading is Chanaud's Pusser's Island Thread. In my limited experience here, it is the best open thread I've read with the best collection of writers.
What an amzing collection of such deep four dimensional characters have collected on this tiny island!