I guess if I had to pick just one fly to fish with, it would have to be the Woolly Bugger. I could use it in lakes, rivers, streams, and saltwater. I could fish it with a floating or sinking line. I could fish it as a baitfish imitation, a nymph, a damsel, koura (NZ freshwater crayfish) – you name it and a Bugger will catch it.
Midcurrent have an excellent article by the creator of the Woolly Bugger, which provides some valuable insights into the way this wonderful fly can be fished. Of particular note is the section on fishing the Woolly Bugger upstream as a nymph – too many anglers see the Woolly Bugger as a downstream fly.