5. Caterpie
There is a reason for which I've chosen Caterpie to the other low level bug types in the series. I believe that this one is the weakest of them all. Up until generation 4, Caterpie would only learn Tackle and String Shot. This would make him absolutely useless against mostly everything, especially ghost type. Generation IV came along and brought Bug Bite with it, a bug type move that Caterpie can actually use. I don't see what the pokemon could actually bite with, but it's pokemon, where there's a will, there's a way.
