I have completed the entire game, 100%, and unlocked every attack.
I have to say that a full gun mage comes a very close second, but Samurai wins hands down. Only a couple of
bosses are immune to Zantetsu (Instant kill), and fireworks is very nice.
Also, when you've played for as long as I have, momentum really starts to get going (3000+). If you do not know the full function of momentum, it does more damage the more enemies that character defeats.
I personally do not like the dark knight dressphere, it is much to slow even with haste.
Trainers are always good because of hp/mp stroll. Mascot is good for a while, but eventually becomes obsolete as characters level up.