Finding all "essential" packages with apt

You can list all essential packages using aptitude search '?essential' or dpkg-query -Wf '${Package;-40}${Essential}\n' | grep yes.

Additionally there are also several packages with priority required that you shouldn't remove, too: dpkg-query -Wf '${Package;-40}${Priority}\n' | grep -E "required"

For your information, Essential and Priority are fields in the control file of a package.

aptitude search ~E

for other search patterns you can look in aptitude doc.