General:
* Valuable
* Precious
* Worthwhile
* Useful
* Beneficial
* Essential
* Important
* Invaluable
* Prized
* Desirable
More specific:
* Effective (if referring to something that doesn't work)
* Meaningful (if referring to something that lacks meaning)
* Significant (if referring to something that is insignificant)
* Profitable (if referring to something that has no monetary value)
* Durable (if referring to something that is easily broken)
The best antonym for "worthless" depends on the specific context in which it is used.