My favorite storyline, its hard to say just one with DS and that's part of the charm of the show. My first reaction is with many of you in the "Quentin's Ghost
and the Werewolf"
storyline. One big reason is like many of you I was watching this when I was roughly David's age on the show, and the David/Amy realtionship was one I could relate to. I also loved Collinwood itself as a character on the show and it was never more mysterious than during Quentin's ghost. I wanted to play there with those two so much. This story was also featured on some of the "Green series" original gum cards.
After that I love 1897 up until the mind switch which gets a little long, 1840 most especially during the Judah's head story, the Leviathans (yes, some people don't hate this one, its pretty good Lovecraft) but I have to admit whenever I try to explain why I love the show and what its about, I usually end up discussing the 1795 story, which I think all things considered, is the definitve story. While it has some flaws, (neither Jeremiah nor Peter Bradford look authentic to 1795 although Joshua, Abigail and Ben do, and does Nathan only have that one navy dress suit?), it has the heart of the show, the romantic triangle of Angelique/Barnabas/Josette, with more straightforward supernatural themes (witchcraft, vampirism rather than reanimated life
,parallel lives, life force switcing, etc. which are cool, but more complex). I think the characters were wonderfully written and acted from the way Barnabas and Joshua played off each other, Millicent's story, and the great friendship of Barnabas and Ben. The conflict with Trask, while not really accurate to the 1790s, still makes for a great nemesis for Barnabas.
So like I said, hard to choose one, but if I had to Quentin's Ghost # 1, followed by 1897 & 1795.