I think Jonathan Frid, Lara Parker, & Grayson Hall played their roles so you could see that... Actually, Jonathan lifted the show, i think -- the other actors improved when Frid was around. Same with Diana Millay, when she arrived. They really lifed everything.
I do think that, after awhile, Alexandra Moltke and Kathryn Leight Scott rose to a great level... In their earliest stuff with Frid, it was like they were unsure. Once they learned to act with Frid, it was often magical.
Look at the episode where Vicki is trapped at the old house during a storm. Alexandra and Jonathan are really learning how to act together -- they really do well within that script and that direction. They are both great, there. Alexandra and Jonathan really got to know how to perform with each other.
Same with Kathryn. She's unsure & nervous in the begining (which works, by the way, cause he's a vampire!). Later, she's doing great stuff with him when she's being held prisoner. Watch the episode where she tries, one more time, to "be Josette," but knows she's Maggie. The one where Barnabas realizes she's out of control, saying, "you recoiled!" whenm he kisses her hand.
They do learn to play very well with Jonathan, I think.
[spoiler]Kate Jackson has some nice moments with Jonathan when they marry in Parallel Time, too.[/spoiler]
Donna and Kate might have had more stand-out performances, if they had more chances.