Pay the highest interest rate first. Or pay the lowest principal balance.
You do not increase cash flow, and even make it worse, while you are paying down a mortgage. Your interest expense is less, and your payment is the same. Once you commit to paying, do it. The faster the better.
I started with a $160K @ 5.375% mortgage last August, will be paid off by the end of 2016. It saves ~$8,500 in annual interest, and $960 in monthly cash flow once paid off. That's $11,524 more cash in my pocket every year.