Joana's Horde Leveling Guide
Joana's Horde Leveling Guide is one of the best known guides on the internet for World of Warcraft leveling. Despite the name, the guide has expanded from its roots and now covers both factions from 1-90.
The guide comes with access to detailed videos, coordinates, and maps for each and every zone you'll be leveling in.
My biggest gripe in the past was that some classes will find soloing certain quests difficult, but that's less of a problem these days.
+Covers all starting zones
-Difficult in several areas for many classes
-Not as fast as later guides
You can get the guide by clicking here.
Zygor Leveling Guide
Zygor Leveling Guides came to the WoW guide party late, but since the beginning it has provided very high quality. It has the same level of professionalism and quality that my old favorite iDemise did, and for a while was my favorite. The Dugi Guides have since taken that place, but there aren't a bad choice.
-Faction guides are separate
Get it here, by clicking this link.
Dugi Guide's Ultimate WoW Guide
Dugi Guides came out a decent while after the first entries to the market had gained popularity, but they certainly made up for time lost with their incredibly well-made and professional leveling guides.
Unlike other guides on this page, the Ultimate WoW Guide is fully featured with sections on leveling, gold, profession, dailies, and more.
It also uses the power of mods to improve its efficiency, as any self-respecting guide does these days.
If you want best MoP leveling guide, it's either this or Zygor. I'd go with this for all the extra goodies.
+Tons of Extras
+My personal favorite :D
-Costs more if you want guides for both factions, unless there's a sale on
Get it now, by clicking here.