I think, within the dream-like concept of Dark Shadows, all the actors do very well. When I first began watching (way back when), I remember being quite annoyed with David Henesy and Sharon Smythe. They both seemed to be caught up in "stage mannerisms" designed to show that someone is a "kid." I still see that when my nieces watch "Barney." All the kids performing there act sooo precious, constantly shrugging their shoulders and moving their arms out from their bodies, as if to say "I dunno!" Henesy, especially, had that annoying stage tic, which he'd thankfully lost (or managed to subdue) by House of Dark Shadows.
The best actor on DS, IMHO, was Thayer David. His Bent Stokes was wonderfully done and a distinct difference from Eliot Stokes. I also enjoyed Kathryn Leigh Scott's Josette, though I never quite accepted her as a frenchwoman! To me, Jonathan Frid was much more effective as a villain...when he became Sherlock Holmes to Julia's Watson, I lost all interest in him. As for Grayson Hall, well, I've come to appreciate her talents over the years. She is greatly missed! I love the way she brings a slightly over the top quality to the show...it fits in beautifully.
For under-rated actors/actresses, certainly Joel Haskell was never truly appreciated by fans, critics, or viewers. He was startling as Nathan Forbes and always projected strength with just the right touch of weakness. When he left Dark Shadows, the show was never the same.
And one last thing--people are gonna hate me for this, but I never did warm up to Alexandra Moltke as Victoria Winters. She always struck me as insincere and amateurish. I can't quite put my finger on it, but I don't believe she ever really grew as an actress or a character. Still, though, her presence on DS was an anchoring force, and when it was removed, a new, sadder era was born!
Rod