Neutral:
* Stated
* Declared
* Remarked
* Explained
* Noted
* Mentioned
* Added
* Commented
* Observed
Informal:
* Went on to say
* Chimed in
* Pointed out
* Added
* Put it this way
* Responded
* Asked
* Whispered
* Shouted
Formal:
* Articulated
* Asserted
* Averted
* Conveyed
* Enunciated
* Expressed
* Pronounced
* Utterred
* Voiced
Figurative:
* Whispered
* Boomed
* Cried
* Muttered
* Gushed
* Snorted
* Squealed
* Hissed
* Spat
For specific contexts:
* Suggested: When proposing an idea.
* Argued: When stating a point of view forcefully.
* Promised: When making a commitment.
* Threatened: When expressing a warning of harm.
* Confessed: When admitting to a wrongdoing.
Remember: The best synonym will depend on the specific context and tone of your writing.