There’s a Horde quest in Azshara that will give you 30,000XP in one hour. It’s the easiest you will ever get. At least before level 60.
When you arrive in Azshara, travel to Valormok and, if you haven’t already, get the Flight Point. Valormok is at approximately 22,50 on the map. After you get the FP, talk to Jediga. She will ask you to find several tablets located about the Ruins of Eldarath.
Once you have the quest, head to the road and then down into the ruins. By the way, the ruins are swarming with level 48 Naga so you will need to take care of a few. There are two types of Naga here, a warrior class and a screamer. The screamers will try and silence you, which can be dangerous for spellcasters.
Look around for the tablets, they are scattered around the ruins, but are easily seen because they are brightly colored. It doesn’t take long to do this quest. You will need to avoid most of the Naga if you want to do this quickly, and you’ll have to kill the ones guarding the tablets, but it shouldn’t take longer than 15 minutes to gather them up.
You will now have the Tablets of Markri, Sael’hai, Jin’yael, and Beth’Amara. Take them back to Jediga. You should get about 5100 XP.
Jediga will now ask you to deliver the tablets to various individuals. You will need to take them to Xylem at the mage tower in Azshara, Magatha in Thunder Bluff, Jes’rimon in Orgrimmar, and Andron in the Undercity. Each of these deliveries will get you 2550 XP. You’ll also get a note to return to Jedriga. When you return to her and give her the notes, you will earn 3800 XP for each.
Oh, and as a bonus, you’ll get paid for these quests as well. All up you should come out with around 4 gold. Not bad for an hours work.