The Parachute Adams is the dry-fly default. Gray body, grizzly hackle wound around a vertical white post, and a low-riding profile that imitates mayfly duns of any species the trout are eating. Sizes 12–22 cover most water.
If you can carry only one dry-fly pattern, it's this one. The high-vis post makes it readable in shaded water; the parachute hackle keeps the body in the surface film instead of perched on top.