Pivotal
Pivotal, key, decisive, determining, testing, searching, significant, momentous, weighty, important, epoch-making, life-and-death, critical, climacteric, deciding, settling, trying, major, influential, consequential, big, historic, and far-reaching.
Very important
Very important, of great consequence, critical, pre-eminent, all-important, vital, indispensable, urgent, compelling, needed, requisite, of the utmost importance, of the essence, high-priority, paramount, essential, vitally important, mandatory, pressing, necessary, and required.