Not all classes are given the same advantages in getting groups. It also depends on the player and what time of day the player chooses to be online. For example, you’re most likely to find a powerlevel group on a Saturday morning. AFK groups are most common at night. And if you dual box, well… you have completely control over your character’s leveling speed.
So this is a list of classes that if you were to return to the game and only used a single box, could level you to 60 the fastest, aka, find a group the fastest. These are all based on my own experience since January.
Ranking (Parentheses are fastest leveling/cming levels):
Damage/Batteries 
1. Alchemist (20+, with aoe pots)
2. Wizard (20+)
3. Bard (8+, mana path)
T4. Mage (45+, afk)
T4. Necro (45+, afk)
6. Enchanter (60, spellbinder with toxic breath)
T7. Ranger (60) 
T7. Monk (60)
Healers
9. Shaman (51+, avatar)
10. Druid (none, raid only)
11. Cleric (60, EG only with resists path)
Tanks
12. Shadowknight (60, with goad AR reducing taunt)
T13. Warrior (none)
T13. Paladin (none)
Note: Group at 60:
With all of this said, the fastest leveling group I have ever been in was an SK, enchanter (animator), and two alchemists using AR pots. In this set up, the SK uses his AR-debuff goad taunt, raising the alc’s pots to 3.3k damage, the enchanter’s nukes to 3k, and the animator proc well above 2.5k. A bandit henchman dropped every 4-5 seconds for 1.7 to 2.1 million xp.
Enchanters have a CM for an AR debuff that works really well in a group with Alcs also.
Yeah so replace the SK with another alc? If you were somewhere using toxic breath, I’m not sure the mob would live past 2 seconds.
Yeah, a couple alcs do well with it. I don’t think the damage output would be as high, but even a necro spamming lifetaps hits really hard with that debuff.