indispensable - adjective
Example: His putter was the indispensable club in his golf bag.
"indispensable - correct spelling." Grammar.com. STANDS4 LLC, 2017. Web. 20 Nov. 2017. <http://www.grammar.com/indispensable-correct-spelling>.
It’s now more important than ever to develop a powerful writing style. After all, most communication takes place in reports, emails, and instant messages.