Let me preface this by saying that I don't dislike any type. If properly used, even the worst type can be an advantage. Now what is the worst type in Pokemon? To me, that distinction belongs to...

The Bug type!

The Bug type is tied with Grass for the most resisted type. And for those types who are weak against it, I'd rather use a Fighting type for Dark types, a Fire type for Grass types, and a Dark type for Psychic types. And look at all those Pokemon with the 4x weaknesses! Yikes.