I'd probably say Normal-type. It can't hit any Pokémon super effectively. I don't know about you, but I enjoy my super effective STAB damage. It's what wins battles. However, at the end of the day, I don't really judge types as being bad for the most part. All in all, it really depends on the Pokémon rather than its type. For instance, even though many people might say that Bug is the worst type, Pokémon such as Volcarona, Butterfree, Scizor, Yanmega, Scolipede, Heracross, and so on are some of the best Pokémon you can use. Also, even though I picked Normal as the worst type, there are still many Normal-type Pokémon that I really enjoy using, such as Tauros, Ambipom, Ditto, and so on. Yet, there is a reason that Normal-types are highly underused, and that's because of what I said in my second sentence.