At any given time, some flies are better than others, but I don't think there are really any bad flies. IMO, not being successful with a certain, popular fly is generally a matter of not using the fly at the right time / place or not having confidence in the fly. The Prince nymph, for example, is a very popular fly and I know flyfishers who use it quite successfully. However, I've never had much confidence in it and don't catch many fish with it.