Warning: Long Post

Where you are now, you have the Warrior, Thief, Monk, Black Mage, White Mage, and Red Mage. I suggest one or two Warriors, a Monk, a White Mage, and maybe a Red Mage. Black Magic isn't all that effective in that stage of the game. Be warned, in Hein's Castle, black magic is almost essential to beating the boss.

Right after the Tower of Owen, you're very close to getting your second compliment of classes. Use the Geomancers. They are incredibly good. Even if you only use them to attack, they're really good. They're mostly treated as a mage class, but they're a very effective fighting class. the Terrain skill has the issue that its random, but it can be effective in certain circumstances.

(SPOILER)Near the end of the game there are actually three classes that can equip the best weapons in the game. It rather disappoints me that there isn't a second powerful long range weapon. But I digress...

The Knight can equip Excalibur and Ragnarok. Until this point I have found little use for a knight. I'd be just as well off with a caster or another melee class. About the knight becoming better than the Warrior, I don't know, but it can certainly equip better weapon in late-game. The warrior does much more damage than the knight mid-game. Especially with his Advance ability.
The Ninja can equip Masamune and Moonring Blade, as well as any other Dark Blades, including the Kiku, which has a higher attack than the Moonring Blade The Dark Knight can also equip Masamune and Kiku. He cannot equip the Moonring Blade, and can't throw Shurikens, but he can deal with groups very well using his special ability.

As for casters, I used a Devout and a Sage at the end game. I pretty much knew the sage would go last due to his low speed, so I planned accordingly. I never used Summons, mostly because I could only effectively use them so late in the game that I was needing two healers at that point anyway. Or at least, I felt far more secure with two healers. Two casts of Haste is very effective in the first round. You could probably get away with only using one healer and relying on elixers, which by that time you could reasonably use anyway.
A group of three Ninjas would be interesting... three shurikens per turn... vicious.

Realistically, the easiest party to beat the final boss with is probably Devout Sage Ninja and Knight. You get the uber weapons of doom with the Ninja and Knight, you get the Shurikens, and the Sage and Devout can either haste or heal every turn, thereby allowing you to continue fighting a long fight.