The Hare's Ear may well be the "standard" in mayfly type nymphs. This one adds a bead and some soft hackle making it even better!
It's called the Guide's Choice. You can't go wrong....you could probably fish for trout the rest of your life with this fly only. We also use it for Steelhead in the 12 and 14 sizes.
The Guide's Choice Hare's Ear is available here in sizes 12-16