What works for 1 site more then likely wont work for others. That's why if you are going to pretend to be a hacker it just pays more to actually know what to look for rather then depending on others to give you a blueprint on it.

But to somewhat answer the question, sites that usually have some sort of function that allow a user to query the server with information are the door to a SQL injection, so basically any site that has a search function is a decent start