This is a difficult question to answer because there are way too many variables that need to be considered. Let me start by listing my recommendations by season.
1. Winter - If you want to catch trout in the winter, there are several ice fishing and stream options. The Boise River is one of my favorite winter stream angling locations because of it's mix of wild and hatchery rainbow trout. The best fishing is from Lucky Peak Dam to Eagle, Idaho. If you want to catch-and-release fish for rainbow, the South Fork Boise and Big Wood River are both exceptional winter fisheries for rainbow trout. Both of these systems have gear restrictions (no bait, etc.).
2. Spring - When the ice goes off Salmon Falls Creek Reservoir anglers catch 15"+ rainbow trout from the bank. Bait or lures seem to be just as effective. Salmon Falls is located along the Nevada/Idaho border.
3. Summer - You can't beat our mountain lake fishing for rainbow and cutthroat trout. Fishing lakes in the Sawtooth Mountains yields breath-taking scenery with great fishing. You can hike for a day or week into this high elevation location.
4. Fall - Fall yields the most options for trout fishing. If you want to fish "flat water" consider C.J. Strike Reservoir on the Snake River. If you want stream fishing, the Upper Snake area north of Idaho Falls, Idaho is some of the most scenic country in the state and fantastic fishing. I'd also suggest the St. Joe River in the northern part of Idaho. Fall colors and an abundance of cutthroat make for fast action.