Gates McFadden was absolutely horrible. Like watching driftwood try to emote. And a strong second place to to go to Robert Beltrane from Voyager. Wow. If anyone could sleepwalk through an episode, it was him!
Notable mention goes to Marina Sirtis, Avery Brooks ( who was inconsistent...sometimes very good, sometimes very bad) the actor who played Kes in Voyager, and Scott Bakula (another inconsistent actor...sometimes great, sometimes bad).