However, it's important to consider the context:
* For a person who is following someone obsessively: "harceleur/harceleuse" is the most accurate term.
* For someone who is constantly contacting someone unwanted: "persécuteur/persécuteuse" might be more appropriate.
* For someone who is spying on someone: "espion/espionne" would be the best choice.
It's also important to remember that the term "stalker" has a very specific meaning in English, and it's best to use the French equivalent that best suits the context.