General:
* Magpahayag - This is the most common and general translation.
* Ipahayag - This is similar to "magpahayag" but emphasizes the action of announcing or making known.
Specific contexts:
* Ipahayag ng publiko - This means to "declare publicly" or "make a public announcement".
* Ipahayag ang digmaan - This means to "declare war".
* Ipahayag ang pag-ibig - This means to "declare one's love".
The best translation for "declare" will depend on the specific situation and the intended meaning.