Define "essential". All boards are different and all of them require different stuff. If there are particular things you need, just look in the RM if you can find that addon. If not, well, just look what the most popular resources are in the RM. It also depends on if you can spend money or not.
Anyway, as a start, free resources sorted by ratings for XF2:
https://xenforo.com/community/resou...ype=free&order=rating_weighted&direction=desc