Ok, I'll play. Consistent with my contrary nature, I will pick Naggie. (I was going to BEFORE I read I was out-numbered, nyeah...) Not because she is such a grapefruit, but because she is the newest to the house. I consider living at Collinwood to be a real detriment to anyone, and the worse for the length of residence.
Of course, I would believe Mrs. Johnson also. What's not to believe from a spy turned loyalty itself? Also, she is the next to newest person currently residing at Collinwood, I think. Julia might have her beat though. Repressed hysteric, my eye!