The majority would say NM hunting, but that proposes many issues if the NMs are limited. For the sake of the argument let's say you're in a perpetual un-timed instance were you hunt NMs til you're done. To make this challenging enough to be more entertaining than grinding lowbies it would need to be difficult and would therefore lack flexibility when it comes to the amount of players and the party setup. Again for the sake of the argument, let's say the jobs will be balanced so well that there will be no ideal setup to do this. And putting a group together would take no over the top thought or planning. Then this is in effect is just a form of grinding with bigger numbers (in the most ideal situation). Not considering the drops.
I would suggest combining the two so that if you you were at a camp with a party, after you've killed a set X amount of the same type of mob. A mob would spawn that would look no different than the rest you're killing but have x20 the HP and a few more difficult to deal with abilities. Beating this mob could grant you a slew of rewards e.x. double exp for X amount of time, double spirit bond, a crazy party buff, something to enhance the gameplay. No drops to eliminate competition of superior camps.
The trick being to find a balance of being as efficient as possible while retaining the ability for a long intense fight without knowing when it's coming until you're already engaged. It could even give that leve animation when the monsterchanges from one thing to another.
All of the "Buried" NMs (BNMs) could also have a one time key item drop that would unlock fighting the creature in its hyper mode in a battlefield you can be prepared for that rewarded with a chink exp reward.
That would be my ideal leveling experience evoid of all the little cool stuff you could do on the side. Which conveniently isn't the topic of discussion.