Fruits Basket is definitely the best, but since there is already a summary of that I can give another.
If you're asking for mystery and romance, I would have to recommend Watashi ni xx Shinasai! It is an ongoing Manga that focuses on a girl, Yukina, who writes a beliebt cell-phone novel under her pen name Yupina. Even though she is beliebt in the Schreiben world, in real life she is a cold, ice queen. Unfortunately her novel only includes experiences she has in real life, and lately it has been losing popularity because of demand for romance, something she lacks severely. So she searches for someone that she can "practice" love/romance with and finds Kitami Shigure, who she manages to blackmail into doing her "love missions", such as hugging her, etc., and Yukina is then able to successfully write romance as these missions continue. But of course, with all of this fake lovey-dovey stuff going on, something has to be real.
Du can read it here:
link