Not really that much of a challenge. Used properly, the starter classes are very powerful. Archer gives you access to Concentrate, which I still believe is the best Support Ability in the game, and you can get well balanced skills and powerful equipment even in the beginner jobs.

Human - Soldier, probably. All around decent stats, self heal, and decent abilities. Add in Thief as sub abilities.

Moogle - Thief. With Animist sub abilities. Great speed, and quite a bit of powerful status effects, not to mention Cuisine.

Bangaa - Warrior. Much better armor and weapons, and White Monks shine in the ability group anyway, not equipment. Far Fist with a Sword/Broadsword is plenty.

Nu Mou - Black Mage/White Mage. Both classes rock, so take your pick, but I'd pick Black Mage as primary. Looks cooler and gets slightly better weapons (and the Sapere Aude).

Viera - Archer, definitely. Long range shots, with the powerful Aim: Arms/Legs and Concentrate. If you need the healing, go White Mage secondary, otherwise, go Fencer, but you'll probably use mostly archer abilities. Faster rocks.