— Copernican
— all important
— all-important
— alpha
— beta
— big
— burning
— cardinal
— central
— chief
— climacteric
— consequential
— critical
— crucial
— decisive
— distinguished
— earth-shattering
— earthshaking
— epoch-making
— epochal
— essential
— eventful
— evidential
— evidentiary
— fundamental
— grand
— grave
— great
— grievous
— heavy
— historic
— immodest
— impairment
— impart
— impartial
— imperfect
— imperial
— impermanent
— impertinent
— import
— importance
— imported
— importunate
— importune
— imprecate
— impregnate
— impress
— imprint
— imprison
— impromptu
— imprudent
— in-chief
— influential
— informant
— key
— large
— life-and-death
— life-or-death
— main
— master
— meaningful
— measurable
— momentous
— monumental
— most-valuable
— noteworthy
— of the essence
— operative
— outstanding
— pivotal
— polar
— portentous
— primal
— primary
— principal
— probative
— probatory
— prodigious
— profound
— remarkable
— serious
— strategic
— valuable
— weighty
— world-shaking
— world-shattering