I have to agree with MB about 1970PT. I think it's one of the best storylines on DS. It's a simply murder mystery, but with a good DS twist infused in there. Suspicion passes from one person to the next as several people claim to "know" who's killed Angelique, and this leads to several characters' death, who, in RT, have never been killed. The killing of this "mainstream" characters is shocking, even though we know in RT that they are still alive and well.
I like this storyline not only because of it's simple nature, but also because it's so tightly contained. It's not like 1897 that stretched on for 9 months (or some absurd amount of time). It lasted for it's run and was well played by all the actors. And although the absense of many of the shows more popular characters in the beginning of the arc posed a problem because it felt awkward at first, it gave room for Quentin and Angelique to shine brilliantly.