I really dislike most of the episodes that turn into musicals. The hippie TOS episode is dowright painful to watch. I admit I'm not a fan of the Vic Fontaine DS9 episodes either. I'm not too fond of time travel episodes or movies either, although I'll overlook it if the story is otherwise decent. Some exceptions include Doctor Who, where the whole show is built around the concept and the Babylon 4 plotline on B5.