I would recommend using it on Port Piscarilius as it is easily the most annoying city to achieve 100% favour in. In fact, using the Kourend favour certificate on Port Piscarilius, then doing the quest The Queen of Thieves, is a great option for getting the first 30% Port Piscarilius favour out of the way.  Whatever you do, just don't use it to gain Hosidius favour. Talk about a waste of resources.