WordNet sense 1 (urgently needed; absolutely necessary):

vital

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

indispensable

WordNet sense 3 (forming or having the nature of a turning point or crisis):

decisive

WordNet sense 4 (of extreme importance; vital to the resolution of a crisis):

crucial

WordNet sense 5 ((especially of fissionable material) able to sustain a chain reaction in such a manner that the rate of reaction increases):

supercritical

WordNet sense 6 (fraught with extreme danger; nearly hopeless):

desperate, dire

WordNet sense 7 (causing fear or anxiety by threatening great harm):

dangerous, serious, severe, grave

WordNet sense 8 (of critical importance and consequence):

acute

WordNet sense 9 (inclined to judge too severely):

hypercritical

WordNet sense 10 (harshly critical or expressing censure):

censorious

WordNet sense 11 (tending to find and call attention to faults):

captious