Ho do we select Valid/Interesting values for a test case?

I am doing research on automatic generation of test cases based on Junit rules framework. I developed a new tool i.e. automatically generating test cases for Java bean naming conventions(getters and setters). It generating test cases for only those methods. In market we have different types semi-automatic test tools . I tried a different one through JavaBean naming conventions, Reflection API and parameter space partition. It is working perfectly. But i am confused in what type of parameter values have to give for testing methods of given code.
My question: How do we select valid/intersting values for the test case?

I would suspect choosing test cases which can cover as many code execution paths as possible would be a good place to start, but that is by no means an end-all answer. Creating good test cases is a very difficult problem, even when they are written by people.