The Sculpin in Olive, size #6, from Solitude Fly. A straightforward sculpin imitation in natural olive.
When in doubt, a sculpin pattern in olive is rarely the wrong choice for trout. Keep it near the bottom with a slow, twitchy retrieve.
Streamers imitate the baitfish, sculpins, and other forage that big trout hunt.
Strip them with an erratic retrieve, and don't be afraid to pause; many strikes come on the drop.