quest char

A mage is useful because of mobbing. An assasin is useful because of haste/flashjump for quick movement.

 

A gunslinger

 

This is going to sound crazy, but hear me out-
Bandit/CB. Reasons why-

1. Bandits, with Savage Blow, are the hardest-hitting 2nd job in the game if equipped and built correctly. This is actually important for questing-only characters because about 90% of the quests are for 1st/2nd/3rd job. And most of that 90% is available for 1st and 2nd job levels. You'd might as well be strong AF for the majority of the quests.
2. Yes, it would make sense to have a mobbing class for many of the quests, but at around level 4x or 5x, barely even halfway through the quest list, you're going to run out of quest-related mobs that you can even mess with because your damage will be a joke to them. And all of the ridiculous "kill 999 of these, 2300 of these" etc. quests can be saved until 3rd job when you get a fantastic mobbing skill. No level limit on those. And you'll still be busy until then with other quests to get you there. Again, this is applicable to the *quest-only* route, a very specific scenario we're talking about here, so consider carefully. You will be having to constantly over-level a mage to complete an obscene number of quests that are for lower levels.
3. Bossing quests are just depressing with a mage. You'll feel like a real bossy bosser 6-hitting bosses at level 40-80. Good luck killing Snack Bar, Capt. Latanica, etc. with Magic Arrow or w/e.
4.

Source- raising a mage and a quest-only Bandit 750 quests in.