What can be the best excuse for taking leave?

First, I would say that if there is no pressing issue/s that need/s your addition, then don't go on leave anymore. Because however good a liar you are, there is always a possibility that you would get caught. However, if you really want to, the best excuse to put there is that you have take care of some FAMILY MATTERS. They don't have to know what that family matter is.