WordNet sense 1 (anything indispensable):

necessity, requirement, requisite

WordNet sense 2 (absolutely necessary; vitally necessary):

indispensable

WordNet sense 3 (absolutely essential):

necessary

WordNet sense 4 (of the greatest importance):

crucial, all-important, all important, of the essence

WordNet sense 5 (of great significance or value):

of import

WordNet sense 6 (not suitable to be expended):

unexpendable

WordNet sense 7 (performing an essential function in the living body):

vital

WordNet sense 8 (having a firm basis in reality and being therefore important, meaningful, or considerable):

substantial, substantive

WordNet sense 9 (highly recommended):

must

WordNet sense 10 (constitutional in the structure of something (especially your physical makeup)):

constitutional

WordNet sense 11 (of primary importance):

primary