Depends on if it's for Bass or Trout.
For Bass on the CA Delta, there are HUGE Dragonfly Nymphs. I tie a thumb-sized nymph inverted (hook point up). #2/0 Gamakatsu Worm hook, .020 lead wire from bend to 1/2 way up the shank, with 3 layers for craft foam covered with a wire dubbing brush, living rubber for legs and a tuft of the stiffest deer hair I can find to make a weed guard. From the top it looks like hell. From the bottom it has the right profile. It can be thrown on/in most anywhere I want and I can twitch it back.
For trout I still haven't found 1 I really like. Usually I just tie on a Damselfly nymph.