Most powerful WOW character/class for solo-play?

[Bolty]

Now that the game is going final, and a lot things have been changed and rebalanced, here's again a question I've been asking about a half year ago:

What is the most powerful character/class if you intend to play WOW just occasionally and mostly solo? Is, for example, the melee-centered Orc Warrior ín WOW the same power-house as the Barbarian is in Dibalo II, who was perfect for solo-games? Or should one better choose a Mage, Druid, Shaman or Warlock? Or a Hunter?

And another question:

It has been mentioned that WOW players are allowed to create up to 50 different characters. Are all these characters possible only under one account name, or can you create multiple accounts as in the Diablo II Battle.net?
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I've played Warriors, Priests, Mages, and Warlocks. Of those four classes, the Warlock was the best soloist for me - once it gets its Voidwalker at level 10. Not the fastest killer, but a Warlock with a Voidwalker does not fear fighting 3 mobs at once - with expert pet management, the Warlock can manipulate the flow of battle and take down all 3 without hardly getting a scratch.

The tradeoff of course is killing speed - Warlocks kill slowly and the tedium can get to people.

-Bolty