yep i have to agree when fishing a team of wets on local north east rivers the greenwells is always on my team, however if there was one best fly my fly boxes would be very simple, i have 4 boxes with me in summer( dries, wets, more wets but all spider patterns, then the big guns for dark and sea trout) far too open question, depends on water, time of year, weather, hatch, size of local flies,size of fish, try and match the hatch is an idea, spider patterns (wet flies) will always work on a flowing river fished as a team ie snipe and purple, greenwells glory, partridge and orange , i do extra