General opposites:
* Loud
* Noisy
* Boisterous
* Rowdy
* Lively
* Active
Specific opposites:
* If quiet refers to a person's demeanor, the opposite could be: Talkative, outgoing, vocal, bold
* If quiet refers to a place or situation, the opposite could be: Bustling, crowded, chaotic, lively
The best opposite word will depend on the specific meaning of "quiet" in the context you are considering.