The worst actor among the cast is easily Denise Crosby as Tasha Yar. She might have grown into the role if she had stuck around, but even her return as Sela is rather bad, which leads me to believe that Crosby doesn't really have the best acting chops. She's not always bad, just not consistently good.
Marina Sirtis on the other hand I would consider a decent actor, though her delivery is sometimes so awkward (especially in Season One), but with the put-on accent and everything she really sounds like she's going for her character coming off as something uniquely alien. And it kind of works. She sort of hams it up at times, but that's forgivable in Star Trek, and her performance evolves and eventually makes Troi act more human as the series and movies go along. Sirtis' problem seems more to do with the writing, but she does well enough with the material that she's given, and even that isn't as bad as some people say.
The worst actor in the series apart from Crosby has got to be Gates McFadden. She definitely has the Dr. Crusher/Mom thing down pat, but it really seems to be the same thing for the entire run of the series and movies. I guess she sort of sharpened her performance as time went on, it was sort of ironic that she was the ship's drama teacher, but overall I found McFadden's chops to be very bland and one-noted. It's not like I can recall any scenes where her acting completely ruins the on-screen drama, that rarely if ever happens in Star Trek, but I equally cannot recall a time when her acting ever engaged me from an audience perspective. I would miss Dr. Crusher's presence if she wasn't there (*ahem*), but she doesn't add anything for me beyond a passable performance as the ship's Chief Medical Officer.
