What is the shortest route to a functional fly box for trout in running water? The fishers who claim to know what trout are thinking (inferred from how the fish react to what fishers do) claim that in fly selection, life stage (adult, nymph, etc.) is most important, size is next most important, how the fly moves is next most important. Least important is the color. Quibbling on these points is likely.
If I had only one life stage represented (particularly for caddis and mayflies) in my fly box, it would be the nymph. If I added another life stage, it would be the emerger / cripple. Even though I know they are not the best choice, I admit to liking fishing dries best, to the detriment of my catching. If I were building a fly box from scratch, I'd fill it with nymphs and emergers before I started tying dries.
Which patterns should you choose for your first nymphs? There are a lot of favorites, but I rely on two old stand-bys: Pheasant Tail and Hare's Ear. With two variables, length and thickness, either or both of these flies can be made to look like a lot of the insects in a trout stream. So before you move past your nymph section, tie or buy either or both from size eighteen to size eight. (Smaller and larger can be added when you have your second inning at building a good fly box. Variant ties, such as bead-head, peacock thorax, etc. can come later, too.) Here's a tip. Tie or buy nymphs with a pronounced dark wingcase hump. This anatomical feature is characteristic of mature mayfly nymphs on their way from the bottom to the water's surface to metamorphose into duns. Their vulnerability to trout is never greater than during this activity.
Which emerger pattern is a good one to start with? I've had some great successes with ties based on the Quigley Cripple model. Get them in a range of sizeslonger lengths (not necessarily hook lengths) compared to the nymphs because the partly shed nymphal exoskeleton is represented. A classic, the wet fly, is no doubt often taken as an emerger or a caddis pupa. The soft-hackle versions are making a comeback. Mike McGuire will sing their praises to you.
Now that you've got nymphs from small to large, and emergers from small to large, you add dry flies. Again a range of sizes is more important than different patterns or colors. Get a full set of either Adams (a dark grey) or Light Cahill (a creamy white). When you have a full set of one get a full set of the other. When you have both of those, you might enjoy the results you'll get from a Stimulator. Stimulators can imitate, in the right size and color, caddis from small to October, stonefly adults, and grasshoppers. This is a versitile fly.
A basic fly box would not be complete without a fly (disdained by tiers because it is so easy) that can truly be called a jack-of-all-trades: the Wooly Bugger. Good in running water and still, good for many species of fish, make some room for this one. Is it a stonefly nymph, a leech, a crawdad, a dragonfly larva, a minnow, a sculpin, a worm, a caterpillar, a damselfly nymph, a chunk of rotting salmon flesh, a hellgrammite? In the right size, the right color, and fished the right way, it plays many roles. Start with a variety of sizes in black. You'll lose many to surprisingly violent yanks.
I doubt many would argue with the basic premise of a full range of sizes in a nymph, an emerger, and a dry, but all would recommend switching some fly pattern in this list for something else that they use, with great success. Furthur complicating matters is that running water is not all the same. In infertile high elevation streams, the fish might hit any attractor that remotely reminds them of a food item. In fertile flat water, especially during a hatch, accurate imitation of the size, shape, color, and movement of the bug-du-jour could be absolutely necessary. That is why you won't stop filling fly boxes when your basic set is in your kit.
Hey! All you veterans. Have you checked your boxes since the end of last season? I'll bet you need to replenish your supply of the starting line-up.
I'm not done with you, Veterans. Add your opinions. While you're at it, how about some recommendations for flies for the next stage beyond the basic level. What attractor patterns do you favor? What species-specific ties do you like. Are bead-heads gotta-have flies? What about the magic effect of peacock herl? Some words on the when, where, and how to accompany your recommendations would be welcomed.