Well, I can't watch TOS without laughing, but that may be because I find the idea of a starship mostly controlled by toggle-switches hilarious. Then, of course, there's Shatner's acting and pretty much every fight scene ever in the entire show.
Pretty much any time Deanna Troi shows emotion in TNG is unintentionally hilarious. Double that hilarity if she's interacting with one of the better actors in the cast (the contrast makes the overacting even more obvious).
I tend to find Worf funny when he's supposed to be serious too.