After a long hesitation, I made my choice for the whale 2015 stone:

I chose her for a rare(for a 5*) +30% ATK on bosses and skill activation. She can be usefull to complete certain teams.

Now, I still have my free 2015 stone from I don't even remember why and my 2016 from the campain... That's even harder to make my choice. So much interesting girls...