I would choose the disappearance of Madeleine McCann, she was just 3 years old when she disappeared, I would use my time and resources to see all the camera footages, with all the technology, I would make the security camera not that blurry and see who that man in the footage had taken her from her room. and well.. if I find out, I would identify him by using the things the CIA uses, you can listen and here things from other people's devices so I would see where his location is and then go arrest him and see if she is dead or not. because that poor girl deserved to rest in peace if she is and deserves to come back to her family if she is.