I don't think that has been asked so far
Well, I haven't seen every single Tv- episode yet.. so from what I've seen I'd say Skip (or whatever this mammal in the 'Starday Celebration' episode is called) because he is just pointless and doesn't add anything to the plot. Besides, I couldn't understand him very well <_<
The second least liked is probably either this creepy crab or Rhett. The crab is just annoying (as they are in general) and Rhett, I probably just hate with a passion
Though he's a character with the possibility of further stories
The worst characters seemingly appear in the Tv-series...