If you are like me and don't like casters because they run out of MP, get the Monk's Brawl ability (15+30 AP), and your casters will do the same attack damage as a Monk if you remove their weapons.

Sticking with a Knight for several levels is also worth it because at lv3 (10+30+50 AP) you can learn Double Grip, allowing you to hold a sword with two hands for double damage.

As for White and Black Mages, you don't gain any benefit from leveling the class past the level you need to cast the magic spells you own.

There are tons of ways you can build your party in this game, so just pick whichever classes you like.