I would pick Mage so MP (DPS), PK (Heals), and KP (Tank). I know the KP isn't the best tank but I love the Mage class and wouldn't give it up.
However, in the new chapter I heard a rumor on an item that allows you to unlock another extra class. If its true my 4th would be a Scout.
This item has existed since chapter 1 (I believe), it's available for purchase in the TW servers, you can have all classes (though it's a lot of work).
Anyways yeah, it's an itemshop item, I dunno if Frogster will consider implementing it though.