WordNet sense 1 (directly relevant to a matter especially a law case):

material

WordNet sense 2 (of the greatest importance):

essential, all-important, all important, of the essence

WordNet sense 3 (of great significance or value):

important, of import

WordNet sense 4 (having the power or quality of deciding):

determinant, deciding, determinative, determining

WordNet sense 5 (determining or having the power to determine an outcome):

decisive

WordNet sense 6 (being of crucial importance):

pivotal