Quickest way to level

[Niwashi]

Your daily Low Level Duty Roulette is the best source of EXP, once per day.

And for tank (or occasionally healer) classes, it can be more than once per day. The big XP bonus that's available to all classes is a once-a-day thing, but the most-needed-class bonus is available however many times you want to run it. If leveling multiple classes, I'd run DR once as DPS, so that class gets the once-a-day bonus, and then run it again as a tank or healer, whichever is currently getting the most-needed-class bonus.