I think the team has done a pretty good job of ensuring that all essential functionality is rolled into the core product. It’s almost the opposite of platforms such as WordPress, where there are certain essential plugins for basic functionality. To that end I’m not sure there are any ‘best’ plugins, although there are many which do do an excellent job of showcasing how Discourse can be extended.
It’s pretty good news to someone like myself, as the act of installing a plugin typically incurs a brief window of downtime while rebuilding the app. There are ways to build out an environment which minimize this impact, but they definitely aren’t recommended when first getting to grips with the system.
There are very few if any common plugins across the 100+ instances which I have built over the past seven years. It very much depends on the use cases that you’re looking to support as to what might be useful.