Since I've driven well, every one of those, more then once, in all kinds of weather (lived there for many years)
MT-200, steep climb west bound from Great Falls to Lincoln, great from then on. Of course then you have to take either I-90 or US-12 to get to Idaho. In some ways the GF to Bonner is the most scenic (the "River Runs Through-It" is this area)
US-12 four lanes over much of the steeper grades, often the preferred route for truckers
US-2, lowest pass (PERIOD) on the Divide, least amount of grades (not NONE, just fewest), then a really easy (if windy) road to Idaho
I-94 (ehh?) Do you mean I94 to Billings then I-90 to Missoula and then over the pass to Idaho. The most heavily traveled, most gas-stations, campgrounds, everything. Good for quick trips, not much fun.