* Necessary: This is the most direct opposite, meaning something that is essential or required.
* Essential: Similar to necessary, highlighting the importance of something.
* Required: Implies a need or obligation.
* Important: Signifies that something has value or significance.
* Useful: Indicates that something has a purpose and benefits.
* Valuable: Shows that something is worth having or possessing.
The best opposite for "unnecessary" will depend on the specific situation you're considering.