I've never been accused of being a one fly guy. I like to have at least three patterns on stream with me, along with three variations of each, making nine different choices. But I can usually select on the first try, a kebari which will do the job. After many years of reading water, lighting conditions and and studying the habits of trout, I can come close to an effective solution on the first try. Because of this, I think anyone should look at this method as using 9 - "One Flies". To me it often feels like nearly one fly for every situation. This article is for the newcomer who has a penchant for doing things in an orderly, quasi-scientific fashion.
Just like everyone who takes up fly fishing without the benefit of experienced advice, I started my angling career with a mish-mash of patterns and tackle from a department store, which were definitely not appropriate to my needs. So if you are a new angler, I am hereby making a pitch for your first outing to be at the elbow of an experienced Tenkara angler. Particularly one fishing on familiar water. And after some initial guidance, if you have a desire to narrow your fly box selection down to a single pattern, you can surely do so and still catch your share of fish.
If you wish, you might learn something from my own strategy of choosing kebari, helping you zero in on your "One Fly". My personal choices would include a small light rooster-hackled pattern which will present near the surface of a stream in average water flows, a medium sized soft hackle that plies the middle stratum. And lastly a larger, and/or heavier fly that will sink quickly to the depths in search of something large with fins.
A Dr. Ishigaki, Futsu or Kurobe style on a light wire, size 12, 14 or 16 hook satisfies the first criteria. It will begin it's career riding high in or near the surface film and eventually sink a bit. It will appeal to fish feeding on, or looking up at the surface for floating or hatching aquatic insects. It will also be ideal for the small to medium sized trout whose diet will largely consist of small bugs.
The size 10 or 12 sakasa kebari is ideal for prospecting in the middle depths, and is likely to be seen by more fish with it's attractive flowing soft hackle. Hen pheasant, partridge or hen chicken hackle are all good hackle choices for this fly. Adding peacock herl will maximize it's attention-getting properties. Adding a wool body will allow your fly to sink faster/deeper as it soaks up water and is an option worth considering.
A size 6 or 8 kebari, and/or a smaller weighted pattern will dive to the bottom where fish are often found early in the season, during high, turbulent or rapid water conditions or in deep pools for mature fish. The Killer Bugger gets top marks for it's abilities to dive for the gold. It's copper wire weighted hook, wool body and marabou tail combine to get down and entice larger fish that are more interested in bait fish or larger insects. Just be sure that your rod will handle a weighted fly before arming up.
In addition, I would tie each pattern in a couple of different colors for use in varying lighting/water clarity conditions. A dark color silhouettes against the surface standing out against the sky. A light color like yellow or white shows up well against darker backgrounds, and is easier for you to see generally. And a medium bright color like red works well in slightly turbid or rapidly flowing water (and is killer for brookies, rainbows, or panfish).
With this selection I'm confident that could successfully lure trout and many other species to strike under most fishable conditions, assuming that I don't bungle the approach and presentation. These particular kebari choices are based upon my own experiences afield, both in research of fish behavior, stream fauna and on-stream fly tying experiments. The results are never guaranteed, but all things being equal, tying imitations of aquatic insects to proportion and incorporating lifelike movements will add greatly to your success percentage.
I like to think that my approach is sort of holistic, since I am studying the stream environment in addition to fish behavior. In my younger days, I spent hours in snorkel gear watching trout feed, and seining samples of aquatic insects in nearby streams that lacked a trout population (chosen for environmental reasons). Finally, comes the fun part, testing "In Stream" over rainbow and brown trout, occasionally with the rare brookie.
So this is the approach that works for me. In analyzing these findings over the years, I could easily choose a "one Fly" if I wished. It will likely imitate nearly the largest, commonest most popular pattern in the eyes of the fish. The so-called commodity fly.
In fact I know of several streams here within an hour's drive in which I would do extremely well with about any fly pattern. The reason being that these streams are mostly limited to smaller sized fish, due to a limited natural food, oxygen and/or limited water supply. These fish are naive, very hungry and eager to try out your offering. Assuming of course that I avoid fumbling in my footing, or presenting to fish that are not seeing my fly due to sun position or other reasons. Think about our own experience of looking in the direction of a low sun, and encountering limited vision. That's the reason for varying your color patterns.
So what things do you consider when narrowing your selection to a single pattern? To start with go back and reread my approach again while considering your own favorite streams and the sort of environmental conditions you experience there. You may be fishing in a slower stream full of deep pools with overhung trees, or a high gradient one with small plunge pools through a woodland. Or maybe something in between. Does it contain mostly rocks, gravel or exhibit a soft bottom, maybe banks overhung with sod and brush? Sample nearby streams of similar character for aquatic wildlife and fill a notebook with descriptions of the critters that you find there. No need to worry about species and Latin names. Just note the size & colors.
Do some internet research for your area. What are other anglers using? In what season(s)? Stream reports are pretty common and useful. An example of what you are looking for is this: a commonly recommended fly pattern that shows up in a number of seasons, not just one. Like the "Midge" pattern for example. Midges are active for many months of the year in some places and year 'round in others. Now that's a pattern that could be useful. I particularly recommend it for headwaters and high lakes.
Do any results of your search look like the drawings in your notebook? If so, that's a good candidate for final choice and a clue, that may eventually be the key to turning you into a "Master of your Stream" with a single fly.
Other patterns that would make my list of top choices where I fish most would include:
An ant pattern or Dr. Ishigaki kebari, both are equally effective in my opinion, depending upon conditions
A "Killer Bugger" pattern, size based upon rod weight versus depth of water
A Takayama sakasa kebari, or Royal Coachman wet fly, useful for it's all around attraction skills
The Incomparable Pink/Oyster Utah Killer Bug, or the red under color Sawyer Killer Bug (preferably with added wool tail)
A partridge and peacock or Takami kebari, just because they imitate and attract lots of different insects
So this is one method you might try, and could become your method too. It should easily help to choose just a single kebari to place your trust in. Obtain a few flies which in your opinion qualify for a final run off and go out and fish them. But don't take too many, and be sure to allow a goodly amount of time for each to see what it has to offer before thinking about changing flies. Trying them out for several seasons is not an unreasonable goal.
Most of the fishing that I personally do is in the headwaters of large and small streams alike, or by canoe in local lakes. Being able to offer the fish a selection based upon the requirements at hand is often rewarded with success. My nine fly patterns are useful indeed. But a single pattern will work too, and if you do a little homework first, will not handicap you in the least.