All classes are needed in there own way! There is not 1 best class, different classes and specs are used for differnet situations.
Normally the one you dont have in your raid team
There is no one class that every raid needs.
dps monks are the only "bad" class right now i would say, all others have somting that might help the raid in some way. And like somone els told warlock is insanly good becouse of so meny multidott bosses + they do good singeltarget
Hunters. Plain and simple.
We have the greatest on the move DPS, and with 5.1, we have zero penalty.
Depending on the spec, we can do some the best burst in the game, along with some of the greatest AOE dps with practically no AOE cap like other classes would experience.
We can provide nearly every (if not all) raid buff and debuff in the game.
We have awesome utility with Entrapment, Ice/Frost/Snake Trap, ability to offtank certain encounters, dispel enrages, the choice between a 2nd CC or a Silence, and much, much more!
We can Feign Death a wipe so we can mass resurrection everyone in the room! Which can also be used as an aggro drop every 30 seconds!
Once we get the ability to actually taunt raid/heroic bosses, and mend other hunters' pets, we will be unstoppable.
I wouldn't say any class is most needed in general. Specific encounters favor specific classes.
If you're looking for what most guilds are looking for I'd say warlocks followed by hybrid ranged dps with off spec heals.
Any class that the raid leader/'s girlfriend doesn't play.
I'm also going to say warlock. I'm amazed each week when I do Sha. We go to summon the slackers and find we don't have a single warlock in the 40 man group.