This is the most famous, popular caddis pattern ever. It absolutely should be in every trout box in a variety sizes and colors. Trout are still fooled by this fly every day, years upon years after it was first tied. Tie it on first when you arrive at the river and the caddis are out (and sometimes even if they aren't).
This can be fished many different ways to elicit a strike. Dead drifted is most common, but twitching it, or skating it can be very exciting. Also try letting the fly get pulled under the water on a tight line and swing it around until it's below you. This fly has a thin tapered body, high quality palmered hackle and wire rib to help with its durability. The elk wing and head are the correct profile and give excellent flotation.