The Flash-back Hare's Ear nymph is a "must have" fly pattern that can represent most of the specie types in the mayfly family of insects. It's buggy appearance and realistic profile is irresistible to trout, making it a great overall searching pattern. In our area, these smaller sizes are the best bet. All types are size 18.
TIP: Try the gold bead as a general searching pattern; the black bead when there are small baetis around and the "no bead" version fished higher up in the water column and/or in very low, clear water conditions.