Well I'm glad you asked, because this simple question has many anwsers depending on how much effort you want to put in!
There are 3 types of random you can use in Isaac, going from the one that requires the least amount of effort to the one that requires the most amount of effort. There is no "objectively best" method, as it depends on your usecase, the size of your mod, and how much you truly care.
For this guide, let's assume your usecase is for adding a random vector to an entity's velocity: