what classes are the most wanted in a high end guild and or groups?

All groups & guilds usually require some of all of the classes.

However there always seems to be a shortage of healers & tanks,

Druids Can be tanks or healers With Dual spec. so there is your answer most groups & guilds like to have druids for their versatility.

Another good class that lots of guilds want even more so than druids. seems to be Priests.

In my opinon tho Shaman & Druid healers are the best.

Shaman are often sought after in Raid groups/guilds as they are said to be the best Raid healers

Hope this helps a little

Druid are popular mostly for their versatility since they can act in any of the 4 main roles (tank, healer, ranged dps & melee dps), however they can only effectively fill 2 roles, one for each talent setup (or possibly 2 1/2 since a Feral Combat Druid could act as dps or tank without having to change their talent setup, but will be more effective in one than the other)
The other two reasons are their Mark of the Wild 'buff' and lastly for their unique ability to revive a party member during combat.

It tends to be less about the class, and more about the role they play, good 'tanks' (Warrior, Paladin, Death Knight, or Druid) and healers (Priest, Shaman, Paladin or Druid) are often in short supply, whereas many people take dps (All) roles.

In larger raid groups it depends on what the fight requires, a mixtures of classes is more versatile, and also has the advantage that fewer people are competing for the same items, for example you wouldn't want to be in a group with 5 Hunters all after the same ranged weapon.