You pay off the lowest mortgage balance first. As Match said, paying some but not all does nothing to your cashflow. If you are young and only have one or two rentals, I'd say pick up more properties now vs paying off one. Build a portfolio with debt, Then as you get closer to pulling the fire ripcord, start paying them off.
FWIW - last month I had some money left over from the sale of our primary residence and paid off my lowest interest rate mortgage. It was at 3%, my highest rate is 4.5% (I'm currently in the process of getting this and all my other's refi'd) , but the balance on the 3% was around 90k. Balance of the 4.5, 300k. I'f i'd of taken my 90k and put it towards the 300k, i'd still have the same monthly mortgage payments. Now I have one less mortgage and am really looking forward to the extra 1k I"ll be getting in cashflow each month.