Formal:
* Reply: This is a very common synonym, particularly in formal writing.
* Answer: Similar to reply, but often used in a more specific context of providing a solution to a question.
* Reaction: Used to describe a person's emotional or physical reaction to something.
* Rebuttal: This implies a counter-argument or disagreement to a previous statement.
* Rejoinder: A formal reply, especially a witty or clever one.
Informal:
* Answer back: This is a more casual way of saying "reply."
* Comeback: This implies a witty or clever reply.
* Retort: Similar to comeback, but can also be used to describe a sharp or angry reply.
* Feedback: This is used to describe information that is given in response to something, often for the purpose of improvement.
Other:
* Acknowledgement: This simply means that something has been received.
* Reaction: Used to describe a person's emotional or physical reaction to something.
The best synonym for "response" will depend on the specific context in which you are using it.