Believe it or not, Bluegill in a local lake here go absolutely crazy for a #10 or #12 Royal Stimulator. Sometimes it's pretty close to the size of the fish that are eating it, but there you go. As I have never seen a stonefly anywhere near the lake (not even in the creek that flows out of it), I'm assuming that the bluegill are mistaking the stimulator for a grasshopper. My second favorite is the venerable hare's ear soft hackle nymph.
------------------
"If we carry purism to it's logical conclusion, to do it right you'd have to live naked in a cave, hit your trout on the head with rocks, and eat them raw. But, so as not to violate another essential element of the fly-fishing tradition, the rocks would have to be quarried in England and cost $300 each."
~John Gierach
My one wish is that when I die my wife doesn't sell my fishing stuff for what I told her I paid for it...