Formal/Official:
* Announce: To make known publicly.
* Proclaim: To declare officially and publicly.
* Pronounce: To state formally or officially.
* Affirm: To state positively and firmly.
* Assert: To state confidently and strongly.
* Avow: To declare openly and frankly.
Informal/Personal:
* Say: To express in words.
* State: To express clearly and definitely.
* Reveal: To make known something that was previously hidden.
* Disclose: To make known something that was secret.
* Confess: To admit something wrong or embarrassing.
* Admit: To acknowledge something as true.
Specific contexts:
* Declare war: To officially begin a war.
* Declare bankruptcy: To officially declare that a company is unable to pay its debts.
* Declare oneself: To state one's beliefs, intentions, or feelings openly.
The best synonym for "declare" will depend on the specific context and the intended meaning.