What's the best used miners for quickest ROI when power cost is 0? I was looking at s9 and so far that seems to be the quickest return on investment at 435USD at 14.5 TH. You should theoretically be in the green at the end of the year with 624USD if prices stayed current(www.cryptocompare.com), after mining fees and initial cost. Do you think the s9 is the best option in this scenario?

Cheers