Edge of my seat during the "Adam" storyline, when both Barnabas and Angelique/Cassandra confronted each other - both as human at last: Barnabas wasn't a vampire at the time, and Ang had just been stripped of her witchly powers by Nicholas Blair. So she arrives at the Old House, sporting cloak and pistol, angling for an old fashioned Collinwood Corral showdown, right out of the Old West (well, West Maine anyway).
Barnabas survived, but each day thereafter, I wondered if, when and how Angelique would return to wreak more havoc.
Honorable "edge of seat" mention to Grayson Hall's superb work in much of the Leviathan storyline. Her Julia Hoffman was so intensely determined to uncover the sinister truth, each day I feared for her life - particularly when she'd go antique shopping.