Hard to know what could be called a essential plugin since Calibre embed itself any essentials features.
The two closest would be
Manage Series and
Quality Check.
Manage Series is not alway usefull (default behavior of Calibre is enough for a day to day use), but it so much handy if you have to manage more that five entry in bulk.
Quality Check is a very big toolbox. You probably won't need all of them, but you're bound to find
something useful in.
Additional usefull plugins should be
ACSM Input and
LCPL Input, that convert/import ACSM/LCPL files into a epub (still with DRM), but their are a bit specific to be "essential".
Also probably
DeDRM to remove DRM, but since it grey area, I will not elaborate.
Else, any other plugins that come in mind depend greatly of you, so just check the
Index of plugins and take what seem fit to you.
Personaly, I would recommend taking a look to:
Search The Internet, because downloaded book don't allway come with complet metadata (I prefer that than the build-in "Download Metadata online").
Comments Cleaner, usefull to clean the book comment, especialy after past the text copy from webpage.
Find Duplicates, explicit. Usefull one day by year, but realy usefull when you need it.
And
View Manager because it allow to quick switch betewen different sorting of your library view.
Also, if if you have a Kobo,
KoboUtilities could be pretty usefull too.