'One of...' oh to have such troubles...
Pay off the loan with the highest interest rate first. That's likely your student loan. Beyond that, your properties are likely appreciating in value whereas your student loan is not.
Lol I didn't mean to come across like that. Its just a flat that I can't sell and want to get rid of (and is depreciating in value due to it being a leasehold / no one wants flats or properties without a garden anymore!!).