If I had to pick only one fly I could only fish with all year long, I would have to choose the Muddler Minnow. I like this fly pattern because it is so versatile, and it seems to produce on big rivers, small streams, and lakes. Popular sizes ranges from size 4 down to size 10 and it doesn't have to be tied perfect to be effective. In fact, I have a friend of mine who once told me that "the worse it looks, the more fish it catches." So if you are tying your own Muddlers and they don't look that good, do not worry they may still catch fish.
The Muddler Minnow is a great fly for Rainbow Trout, Brown Trout, Cutthroat Trout, Brook Trout, Steelhead, and most other game fish. This fly has two major things going for it. First, it looks like a wide variety of fish forage. For example, to the fish, a large Muddler may look like a grasshopper, big stonefly, or even a small field mouse. The smaller Muddlers may look like caddis flies, small minnows, or small sculpins. Second, it can be fished just about any way you want using a dry line or a wet line, dead drifted on the surface, down and across the current, or cast and striping the fly. Don't be afraid to give the Muddler some action. Make it look like alive trying to get away from a predator, or make it look like a big fly trying to get off the surface of the water.