Tsubaki>Akane>Yuuka>Hanako
Hanae and Mari are whatever.
Now, do hear me out why i put Akane above Yuuka.
Tsubaki is a popular tank in a multiple game modes. Useful in pvp at some points, one of the better Kaiten tanks. For a newcutie, just put t4 bag on her and go wild on clearing stages.
Akane is f2p def shredder, next best update with a cheap ex that rapes single enemy defense is sKanna, and we ain't getting rerun of that banner for a bit.
Yuuka as of right now is not really useful anywhere. With evasion tanks getting raped in pvp in current seasons, she isn't really any good there.
Not really useful for much of raid content. Still might get some use in future PVP seasons, but that's a huge maybe.
Hanako is set core.