If I had to rank and rate them:
1. CO (Likely D)
(1.1 ME - if Collins retires (Pure Tossup))
2. NC (Pure Tossup)
3. MT (Pure Tossup)
4. GA (Tilt R)
5. AK (Tilt/Lean R)
6. IA (Lean/Likely R)
7. WV (Likely/Safe R)
Daines' 2014 margin was an outlier (think of something like MO-SEN 2010) and he will be seriously targeted in 2020. He's not that popular, has little crossover appeal, his voting record can be attacked pretty easily and there will be lots of other hotly contested statewide races on the ballot (especially the gubernatorial race). Plus, the Democratic nominee will probably do better than Clinton here.
Races the Democrats definitely shouldn't target: SC, TX, ME (with Collins), KY and almost certainly SD as well
I think you're a little too bullish on Joni Ernst and a underestimating the remaining 6 Republicans on that list.