Most wanted classes and best places to gear starting at 70

[Ryuudin]

As title says, which are the most valued/wanted classes at the moment and where is a good place to start gearing once you hit level cap?

[Whiksey]

All classes are valued and wanted. But also, all classes are derided and shunned. There are over 10,000 players playing 9 classes, so there's no overarching need for anything. Tanking or healing will get you quickest 5-man pug invites, but joining a guild and making friends is a vastly superior method of getting groups.

Read some wikis and determine what playstyle you like, choose a character based on that.

All of outland is viable for gearing at level 70, presuming you're playing on the Outland realm. If you're purchasing an instant-70, starting your efforts with Hellfire Peninsula or Zangamarsh is mildly advisable.