Really depends on what you wanna play. One Assist Ninja of each element will be good enough at the beginning.

For all of those, reading this before they reach lvl 56 and the feature opens: you arleady get Mei (Water) for free, when the feature arrives. The system then wants you to recruit Ao (another Water), just refreseh the browser / restart the software and when you enter again, system will not force you to pick Ao anymore and you do not waste your flag.