This takes as a given that using your voice is you most critical single weapon against an assailant. The question is what to yell.
One option that's gained some popular traction is to just yell fire. Even an FBI spokesperson has suggested that on national TV. (Your local fire department, however, may not appreciate it, as they are not the police.)
So far I have seen no evidence that yelling FIRE! is any more effective than yelling PIZZA!
What I recommend to my students is that you yell direct commands to the potential perp. Monosyllables and short phrases, like:
BACK OFF!Violence needs silence. Take charge! Perps are looking for victims they think will be scared and intimidated into silence. WHAT you yell is far less important than THAT you yell. Just YELL.
I DON'T WANT TROUBLE!
STOP THAT NOW!
DIE!!! (favorite of one sexual assault victim's advocate I know)