Simple and direct:
* Yes.
* No.
* Stop.
* Help!
* Wow.
More abstract:
* Perhaps.
* Always.
* Never.
* Everything.
* Nothing.
Single-word exclamations:
* Ouch!
* Amazing!
* Goodbye!
* Congratulations!
Remember: A one-word sentence must be a complete thought. It needs to stand alone and convey meaning without any other words.