The qualification with the most rigorous requirements is the PMP. So on some levels, that would make it the best. It requires a minimum amount of training, requires a minimum level of experience, and tests your knowledge and ability to apply it.
The CAPM has similar rigor, but only tests terminology and definitions and has reduced training and experience requirements, so it is a good stepping stone to the PMP for many.
PRINCE 2 I understand you can take a class and pass the test at the end of the week. While experience will help you pass the test, its not required, making it a less rigourous certification.