I'll go by series:
TOS - William Shatner. His sentence construction and delivery really bugs the hell out of me.
TNG - Diana Muldaur. She was awful. A cheap knock-off of McCoy.
DS9 - Don't have any, loved the series and the cast.
VOY - Robert Beltran. It was obvious he didn't want to be there. Should have killed him off, promoted Tuvok and made Ayala the new Security Chief!
ENT - Jolene Blalock. Her method of doing controlled emotions was just a trout-pout. The character I didn't mind, but her delivery was weak in some places.
I agree with everyone on this list except for Shatner, who IMHO played Kirk to near perfection. The only problem I had with the Shat was the occasional ridiculous karate chop, especially the one he delivered to the Gorn.