The easiest way to add a forum to your WordPress site is by using a plugin such as bbPress, wpForo, or Simple Press. These plugins provide all the necessary features for creating and managing a forum and are easy to install and configure.
On the other hand, the hardest way to add a forum to your WordPress site is through custom development, which requires coding knowledge and can be time-consuming and expensive. Building a custom solution from scratch involves creating a database, designing the forum interface, and implementing necessary functionality. This method may require you to hire a developer or learn to code yourself.
Overall, using a plugin is the easiest and most efficient option for adding a forum to a WordPress site, while custom development is the most challenging and time-consuming approach.