Basically what you have to decide is do you want to play a front loaded class or a back loaded class. In terms of overall power/utility for small groups at level 20/40/60 I would say:
Mage: 10/9/7
Necro: 10/9/8
Enchanter 6/9/10
Wizard: 4/3/2
Cleric 6/8/9
Druid: 8/6/4
Shaman: 4/8/10
Of course for raiding the ranking is very different, maybe something like:
Mage: 8
Necro: 5
Enchanter: 5
Wizard: 10
Cleric: 10
Druid: 6
Shaman: 5
In general all of the awesome solo classes have their abilities simply disabled for raiding because if they worked the encounters would be trivial and you wouldn't need a raid.
I had fun making that chart
[You must be logged in to view images. Log in or Register.]