Originally Posted by
SirCowdog
20-58:
Loch Modan, Westfall, Redridge, Darkshore, and Bloodmyst Isle while queuing for Ragefire Chasm and Stockades.
58-73:
Northrend quests while queuing for Utgarde Keep, Ramparts, and Blood Furnace. TBC quests, despite seeming nostalgic, are literally twice as slow as Northrend.
74-81:
Set your dungeon difficulty to heroic and proceed to solo every TBC dungeon. Alternatively, Karazhan trash clears serve the same purpose.
81-82:
You could potentially continue doing Kara trash clears til 82, but you might be better off doing Serpentshrine trash clears instead.
82-84:
Set dungeon difficulty back to normal and solo ramparts. Some classes can start at 82, others at 83. YMMV
83-87:
Back to questing/queuing for dungeons. Pick Cata or MoP, whichever you're most familiar with. Speed will rely greatly on how focused you are. MoP has the advantage of treasures which can be grabbed along the way for a little boost. Personally I'd stick with questing, since the speed is based entirely on you, whereas a group is random, and you could suffer a lot of wasted time if you get a bad group.
88-90:
Heroic Zul'Gurub. You can clear all the trash(and I do mean ALL the trash, including the fish and gators in the water) for a LOT of experience. Kill the sub-bosses, but do not kill any main bosses. Again, you might struggle a bit right at 88. A lot depends on familiarity and persistence.
90-98:
Use the Timeless Isle portal to Gorgrond to skip the WoD intro scenario. Fly to your garrison and do the quests to set it up. Buy exp pots and start doing bonus objectives and treasures. Potentially buy a 630 weapon from the AH if it's available and not too expensive. This will be more powerful than your heirlooms. At level 96 go to Spires and set up the Inn for another 20% experience.
Note: Some people have expressed difficulty with WoD right at level 90 due to scaling and heirlooms not leveling up correctly. If this is a problem, grab some free gear from Timeless Isle, or simply wait to start WoD until 91.
98-102:
Go get your artifact weapon. Some classes will have an easier time passing the required quests. Go back to WoD and continue clearing treasures and bonus objectives on EZ mode because your weapon is OP.
100-110:
Quest, queue for dungeons. Again, go to whatever zone you're most familiar with. Flying will help immensely. Invasions are technically the fastest, if you can do them.
This has all been tested pretty extensively by many people. There are ways to cheese the leveling process using something called "Method zero", but it's questionable whether or not it's against the ToS. Google it and do it at your own risk, and I expect Blizzard to nerf it somehow soon.