Plugins are meant to cover specific use cases; that's the main reason why they are plugins and not part of the core application. Plugins considered most essential ship with the program and are activated by default. Examples of those are Cover browser, Statistics, Profiles, Gestures, and many more. You can tailor the program to your needs by going through the list under Plugin management and activating/deactivating plugins.
If you are wondering about extra (external) plugins, then only you can decide what you consider "essential." Many of them are presented in this forum by the developers themselves, so you can search for "plugin" here and read the discussion on them. Some are also available here:
https://github.com/koreader/contrib/