Aion leveling guides, but keep reading to find out the whether it's the best or not.
Aion Mastery currently includes both Elyos and Asmodian leveling guides, finished up to level 40. The last ten levels will be finished soon which will make this a really complete Aion guide.
The leveling guides include maps of each area, but unfortunately they don't have any quick reference numbers as found in some other guides I reviewed so far. I used it for about 10 levels and found out it's just as good as other guides on the speed leveling front.
There are a few downsides to this leveling guide: there are no screenshots to help you out on tricky quests, and no links to Aion Armory where you could get some more help about a particular quest. Unfortunately, you'll have to type the quest in manually. There is an occasional tip next to a quest giving you more info about what you have to do but more than often you'll have to find more info somewhere else. Kozen's leveling guide is more detailed.
Aion Mastery also includes a kinah guide which is actually really good. Currently there's over 40 pages of useful info which I doubt you'll find anywhere online, and it's more detailed than the kinah guide in Kozen's or Killerguides' guides. It won't make you a millionaire over night but it's a great resource nevertheless.
The biggest downside to Aion Mastery in my opinion is its price. The complete package (Elyos leveling guide, Asmodian leveling guide and Kinah guide) costs $67 which is pretty high. Overall, I think Kozen's Aion leveling guide is still better and has more additional guides, and is a more affordable choice.