Highway 89, between Gardiner and Emigrant
Montana is beautiful and ever sunny. If one wants to bike it, I highly recommend turning on East River Road at the 20th mile and keep on going from there. It's less busy, the shoulder is safer and you see the valley from higher.
I usually just google the places I go and follow the indications, but it had me going on the freeway for 21miles between Livingston and Bozeman. That was scary and uncomfortable. I will try finding alternatives on my next trip.
They call it Big Sky country, and it's obvious why.