WordNet sense 1 (urgently needed; absolutely necessary):

critical

WordNet sense 2 (not to be dispensed with; essential):

indispensable

WordNet sense 3 (performing an essential function in the living body):

life-sustaining

WordNet sense 4 (basic and fundamental):

essential

WordNet sense 5 (full of spirit):

lively, full of life

WordNet sense 6 (having life or vigor or spirit):

animated

WordNet sense 7 (possessing life):

live, live