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[pickel]

Top 3 solo classes?

[Vindictive]

Hunter, Rogue, Shadow Priest.

Then again ---> warriors, mages, warlocks and druids (perhaps a tiny bit less so) are also very good.

[Krollin]

Warlocks can solo anything in the game, outside of an instance, that can be fear kited.

Beast Master/Marksmen Hunters are simply awesome for levelling fast and their ability to punch above their weight. While not quite up to the power of a Warlock for killing things 1vs1 Hunters are more survivable when things do go wrong and do not need to fear kite things. Poison immune mobs are a pain for Hunters.

Rogues combine the ability to sneak into places avoiding trash mobs with a powerful punch when they need to fight.
Path 1.12 will see Stunlock Rogues being just as viable as Combat Rogues for soloing.

Priests: since their review Priests of any colour make for good soloing. Holy Priests are able to deal some pretty nice dmg. Shadowpriests are still the masters/mistresses of soloing.

Druids: jack of all trades, master of none. Feral Druids have all the tools available to make excellent soloists. While not as spectacular at levelling as a Hunter or as good at putting up a horrendous damage like a Warlock or Rogue they are very, very tough and can take on challenges most other classes would leave alone until later. Big advantage with a Druid is Cenarion Circle Rep.

Mages: the early soloing career of a Mage can be really frustrating. The true glass cannon in fact. It is only once a Mage hits the late 40's and starts feeling the benefit of the higher tier talents + better equipment that soloing a Mage becomes a real blast (deliberate pun). Once at level 60 and with a heavy Frost build and loads of +Frost damage items Mages are capable of doing really crazy things with AoE like soloing crocodiles in ZG.
I have played with mages who are capable of clearing the Jailbreak run solo, I don't know of any other class that can do that.