Formal:
* Answered
* Responded
* Retorted (implies a sharp or witty response)
* Rejoined (implies a continuation of a conversation)
* Countered (implies a disagreement or opposing viewpoint)
* Remarked (implies a comment or observation)
Informal:
* Said
* Stated
* Chimed in
* Shot back (implies a quick or sarcastic response)
* Fired back (similar to "shot back" but more aggressive)
* Quipped (implies a humorous or witty response)
The best synonym for "replied" will depend on the specific situation and the tone you want to convey.