Certain flies seem to be able to almost do it all. Here's a few of my favorites and what they can work for.
Wooly or Krystal Bugger - This is THE lake fly, possibly THE fly period to have if you have just one. I like it in black, olive, and brown, sometimes with and sometimes without a bead. You can obviously strip it in lakes and strip or swing it in rivers. You can also dead drift it as a stonefly nymph or cranefly larva, add weight and bottom bounce it as a crayfish imitator, add floatant and fish it as a cricket in black or as a hopper in olive or tan, float it and strip it to imitate a frog or whatever else may swim on the surface.
Muddler Minnow - This fly is designed to be stripped or swung close to the bottom to imitate sculpins but it works dead drifted as a big nymph, floated as a hopper, and stripped in lakes as a minnow or dragon fly nymph. Also, try floating it and waking it on the surface.
Parachute Adams - This most popular of dry flies can basically imitate any mayfly in the right size and almost always works as an attractor. I carry it in sizes 14 - 20.
Stimulator - This pattern is a great multi-purpose fly. It works great for stoneflies, caddis, and even drakes and other big mayflies but it's sneakiest use is as a streamer. Many big browns have succumbed to the sunk and stripped stimi.
Hares Ear Nymph - Just as the Adams and Stimulators are do it all dry flies, the hares ear in the right size and color can work for almost any nymph, is a fantastic lake fly, and can be floated as well. I carry this pattern in sizes 14 - 20, beaded and not beaded, in flashbacks, and in all colors.
Be creative. You can probably find tons of other uses for these patterns and for others that you have set aside for just one purpose.