I joined just a little bit before the end of the second Meridell plot so I don't have too much recollection of it.
So probably the Ice Caves plot?
I always thought Bori were pretty cute, the Thieves Guild had cool character designs, and it was my first real taste of battling.
A lot of the other plots have kind of just faded into history-- either I missed them due to my hiatus, or they didn't stand out much to me.
(I do remember the Sophie plot having that one annoyingly complicated step that took all kinds of forever, though. That was the opposite of fun.)