Most valuable/needed Class for PvE right now

So I've checked the pages a bit and every time there's no real answer on what is needed the most!
I'm an old, hardcore retail PvE played who excelled in all positions and I really do just want to raid a tad bit again. I don't care which position, I don't care which class. I'll get used to all of them if needed!

So can anyone answer this question truthfully?
Which Class/Position is looked for the most during raids right now? Is it a tank? If yes, which class? Or a heal? If so, which class?
I know I could play Paladin or Druid because they're the most versatile but what if Shaman or Priest Heal is needed the most?
Edit: I'm talking about WotLK on Icecrown
Please tell me!:)

Thanks in advance!

Most needed are RShammy & Rdudu & esp Tanks pala & dk.