Like all classes, it's not the class it's the player.
For leveling enhancement is definitely the fastest. Damage output may not be as significant as elemental but damage duration is almost constant. You'd never have to sit down drink some water or anything. You just go on, when hp's low heal a bit, when mana's low shamanistic rage, once you compare it out enhancement has the upper hand for leveling.
Elemental does have the upper hand in 1v1 pvp, against other classes that is. Their high damage and quick output can bring down the target with speed.