Of course, Zachary Smith was an incredibly obnoxious whiney useless pain in the ass--even child snell saw that, and wondered why they didn't throw the schlub out the airlock. (The real answer, of course, is that the lazy writers had no other way to generate action/conflict when all of the other characters were the noble do-gooders that the Robinsons were...)
Of course, Smith was hardly the only useless character who existed only to get the level-headed leads into trouble (there's never a shortage of lazy writers...) In my more masochistic moments, I like to imagine a series where Dr. Smith, C-3PO, Angel from the Rockford Files, and Gilligan are trapped on a deserted planetoid. Just picture the whining...
You are, of course, allowed to make further nominations to populate this sad, sad moon...