Worst regular: Robert Beltran, he wasn't even trying.
Worst guest star: whoever played Sisko's mother. She was completely wooden.
I think I'm going to have a seizure every time I hear her voice. Like Kramer from Seinfeld
when he hears Mary Hart.
I'm pretty sure the title of the thread is "Worst Star Trek actor", not "Actor whose voice irritates you the most".
As for Nana Visitor, she is one of the best Trek actors.