There is no such thing as essential anime.
The concept of "essential anime" is completely dependent on community.
For an anime club in 00s america, cazador de la bruja, full metal alchemist, last exile and stand alone complex are essential anime.
For italians, dragonall, saint seiya, tsubasa and captain harlock are essential anime.
Within a lolita circle, world masterpiece theater, rose of versailles, sailor moon, shoujo cosette, ouran host club and princess tutu are essential anime.
They're the seed around which a community flourishes, influencing opinions/taste on new/modern works. No list is more legitimate than the other.
The ritual murder of smaller web communities and the spawning of web hubs coupled with overabundance and accessibility to current media has been killing and generating false concepts for at least a decade. If you speak to me of "essential anime", "must-read manga", or whatever else, undetached from a community, know that you're at the bottom of the pyramid of mindless tools that are desperate to look cool and your opinion is worthless.