In Spain the Red Butt Perdigon is possibly the most famous nymph used by competition anglers, they claim it's so safe you can just put this on and go fishing. This fly is very effective in all kinds of rivers, just vary size to what kind of nymhs are present. Red Butt Perdigon is a typical generic fly for all kinds of darker mayfly nymphs and even larger midges.
Perdigons were first developed by the Spanish competitive fly-fishing team but were really popularized by the French. These simple flies has the key triggers the fish look for: Right shape and an attractive colors. Perdigons sink fast due to their shape and tungsten bead, and can be fished alone or together with other flies in a rig.