My favorite five generic classes (best things about them in parentheses):

Calculator (Math Skills)
Lancer (Armor, Spear)
Mediator (Invite)
Squire (Accumulate, Throw Stone)
Knight (Armor, Knight Swords, Break)

I use them all the time, they have useful skills/equipment and they're fun to use. I also use Thieves, but only in battles where there are unique items to steal.