I've had very good luck in the past with Field of Bone arena; then Kurns Tower; then Paludal Cavarens; then Overthere; then City of Mist; then Veksar; then Wall of Slaughter.
From Field of Bone up to Vexar, do not group with the character to be power leveled. Depending on the class doing the power leveling, I would do as follows:
Cleric - the low level toon will use damage shield pots and click on a damage shield. The lowby will then pull a massive amount of mobs to the cleric. The cleric will cycle through all the mobs and cast the cleric spell Mark of ? on each mob. This spell causes the mobs to take additional damage each time they hit the lowby. The spell damage stacks with the damage shield damage. When the lowby gets around 15% to 20% on hitpoints, use the clerics most efficient healing spell to complete heal the lowby. Most of the mobs at this point get really pist about this and will agro the cleric. Make sure the cleric has a supply of Damage Shield pots and that a damage shield is on the cleric. When the mobs attack the cleric, they will die quickly on the damage shield and the lowby will get all the experience.
Shaman - Use the same strat as the cleric minus the Mark spell.
Druid - Beef up the damage shield on the lowby and use the same strat as the cleric minus the Mark spell.
Ranger - Same strat as the Druid, however, find a nice corner to put your face in so that reposte/parry damage is not done to the mobs. If you repost/parry a mob, you will steal the experience from the lowby.
Paladin - Use the same strat as the Ranger.
From Veksar and on, I would group with the character being power leveled. The high level character will now be pulling the mobs to the lowby. A high level and nicely geared paladin can slaughter trains of Veksar Highborns and then the Undead Murkgliders in Wall of Slaughter IF they have the Slay Undead AA's. Using a non-tank class after City of Mist can be pretty tough. High level clerics do get a nice self only proc spell that does nice damage to mobs that hit them, however, clerics do not mitigate damage as well as tank classes and their defensive skill caps are not as high. A cleric will take a bit of a wooping if too many mobs are pulled at one time. I know other classes can solo efficiently, however, I stuck with the classes that can heal in this explanation primarily because I use the heal to pull agro from the lower level character.
Well, that's the strat I've used to level up a few toons. I'd try to incoorperate Hot Zones into your power leveling. Dungeon type zones that already have an exp bonus are great power leveling zones when they are turned into a Hot Zone.