Who is the best tank ?

[Tsukiya]

...

[Mercurial Obscurial]

Have you considered paying someone to play the game for you yet?

Okay, yeah, I admit that's a bit rude, but you always ask these kinds of questions with no context. Not every fight is the same and you don't include what weapon your Miya is using which makes a difference. If you just want a good formation then look at the rank tab while you have the game running and copy whoever is in first place for whatever you want to do.

[Panterich]

... and don`t forget all the stats and skills are available in game.

[INT]

miya katana

[GrailQuest]

I don't feel there are really any "tanks" because there is no taunt. You cannot force the enemy to attack any particular unit, and when you encounter them, they might have a fixed attack pattern that hits a whole row, column, or rear, etcetera.