Any one which isn’t immediately next to a cubicle being used by someone already, that’s which one.
This, my friends, is toilet etiquette.
It’s bad manners to unnecessarily enter a cubicle next to one already in use- how can the existing occupant concentrate on his or her business when someone’s ignorant enough to start making toilet noises next door.
Don’t do it. Follow toilet etiquette and use a cubicle as far away from an occupied one as possible. I don’t want to hear you taking a dump or a piss whilst I’m trying to do the same thing- it makes it go back up again.