Unless you're planning on doing this a dozen times, I think the fastest way to 15 is to get off the forums and get it over with. By the time you discover the most efficient way to get to level 15 (a two hour job, max), you could've already been there.

I only went to 11, but here what I recently did:
1. Mail heirlooms to all the new toons.
2. Run to nearest mailbox and put them on.
3. Join a guild with level 2 Fast Track.
4. Kill mobs nonstop for an hour or so, don't loot, don't train, don't quest.
5. Take care of spells, bags, etc. once you're at your desired level.