Neutral:
* Complain: This is a general term for expressing dissatisfaction.
* Moan: This implies a low, drawn-out sound of complaint.
* Plead: This implies begging or asking for something earnestly.
Negative:
* Gripe: This implies a persistent and irritating complaint.
* Grouch: This implies a grumpy and complaining person.
* Bellyache: This is a slang term for complaining in a childish way.
* Wail: This implies a loud, mournful complaint.
* Bemoan: This implies a complaining about something that is considered unfortunate.
* Carping: This implies a persistent and critical complaint.
Other:
* Whinging: (British slang) This implies a childish and annoying complaint.
* Whimpering: This implies a weak or timid complaint.
* Pester: This implies a persistent and annoying request.
The best synonym to use will depend on the specific situation and the tone you want to convey.