Crappie have relatively large mouths for their size, so I use hooks up to size 4 for them. Standard flies like clouser minnows, wooly buggers, and any minnow imitating pattern work well. Sometimes a buggy subsurface pattern such as a bully spider will also get them.
I have always, always done better with crappie when using bright attractor colors such as white, yellow, chartruese, and pink in my flies. These color schemes really seem to get their attention around here, but then again I don't fish in super clear water.