How much higher are your itemized deductions than your standard deduction? If they're pretty close, then you might want to focus on the rental houses since getting yourself under the standard deduction in your personal houses will effectively make those mortgages non-deductible.
Have you considered refinancing any of these mortgages? You could probably knock 1-1.5% off any of these by converting to a 15-year loan. If you're planning to make higher payments on one or more of these loans anyway, you might as well take advantage of an opportunity to reduce the interest at the same time.
I agree with the second part, about refinancing. But, I think your logic is backward on the deductions.
The rental mortgage interest is a business expense so 100% is deductible - or counted against income.
The residential mortgage is only partially deductible (only to the extent that it is above your standard deduction).
Let's pretend you have a primary residence and a rental property. Same interest rate, and each one costs you $15k in interest per year. No other deductions. Assume standard deduction is $12k, and income tax is 25% marginal rate to keep the math easy.
Scenario 1 you pay down your primary residence resulting in $5k less interest on the primary. You pay a total of $25k in interest for the year. You get your 12k standard deduction, nothing extra for your primary mortgage, and $15k 'deduction' for the interest on your rental= $27k in 'deductions', or about $6750 in tax savings.
Scenario 2 you pay down your rental to the same extent. You get 15k in itemized deductions and only $10k in deductions against your rental. This is a total of $25k in deductions or $6250 tax savings.
There may be other arguments for paying down the rental, but tax wise it is almost surely less beneficial unless the interest rate is higher.
