For an all purpose nymph I'll have to go with a Gold Ribbed Hare's Ear in size 16 or more likely an 18. But recently the first nymph I tie on is Davie McPhail's Catgut Caddis Pupa. Terrific fly.
This is a picture of it soaked - McPhail's tie, not mine. I tie mine in bright green.
Joe