What is a 9 letter word meaning the opposite of useless?

There are several possible Words:

For useless meaning valueless, the opposite could be "treasured."

For useless meaning not workable, the opposite could be "effective."

For useless meaning not required, the opposite could be "necessary."

For useless meaning worthless, the opposite could be "practical."

For useless meaning inappropriate, the opposite could be "pertinent."

For useless meaning unhelpful, the opposite could be "desirable."
useful - look it up in a dicitonary and you'll see a picture of me.