Critical
Part of speech: adjective
Definition: Inclined to find fault or criticize; fastidious; captious; censorious; exacting. Pertaining to, or indicating, a crisis or turning point. Extremely important. Relating to criticism or careful analysis, such as literary or film criticism. Of a condition involving unstable vital signs and an unfavorable prognosis. Likely to go out of control if disturbed, that is, opposite of stable. Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes self-sustaining.
Example sentence: Being a good listener is absolutely critical to being a good leader; you have to listen to the people who are on the front line.
Important
Part of speech: adjective
Definition: Having relevant and crucial value.
Example sentence: It's fine to celebrate success but it is more important to heed the lessons of failure.