crucial

(adjective): Essential or decisive for determining the outcome or future of something; extremely important.
(adjective): Cruciform or cruciate; cross-shaped.
(adjective): (Bermuda) Very good; excellent; particularly applied to reggae music.

A secure supply of crude oil is crucial for any modern nation, let alone a superpower.
Delbert Wilkins is the most crucial pirate radio DJ in Brixton.