It's not a type, but the skill is way more important. Of course, there are types that are easier to build a rteam around, such as Water or Bug, while others seem to have less useful members like Ghost or Electric, but all it's a matter of skill. In 4th gen, I ran a SS Monodark, a monopoison, and a hyper offense Monosteel. All had success in OU. I'd say however that the easiest to abuse types are Water, Steel, Ground and Fighting.
Which code?