I'd just like to add the note that Balance druids actually are better at creating threat than Restoration druids, they just don't have the same mitigation. Yeah, a Balance druid can tank via DPS, assuming people are careful. So I'd probably put them equal.
My own "ideal" tanking list:
1. Restoration druids, Balance druids, Retribution paladins, Holy paladins.
2. Protection/Retribution paladins, Protection/Holy paladins, Feral/Restoration druids, Feral/Balance druids, Arms/Fury warriors, Fury/Arms warriors.
3. Arms/Protection warriors, Fury/Protection warriors.
4. Feral druids, Protection paladins.
5. Protection warriors.
EDIT: Changed it a bit after some additional thinking.
My own "ideal" tanking list:
1. Restoration druids, Balance druids, Retribution paladins, Holy paladins.
2. Protection/Retribution paladins, Protection/Holy paladins, Feral/Restoration druids, Feral/Balance druids, Arms/Fury warriors, Fury/Arms warriors.
3. Arms/Protection warriors, Fury/Protection warriors.
4. Feral druids, Protection paladins.
5. Protection warriors.
EDIT: Changed it a bit after some additional thinking.
Earthen Ring-EU:
Taelas -- 60 Human Protection Warrior; Shaleen -- 52 Human Retribution Paladin; Raethal -- 51 Worgen Guardian Druid; Szar -- 50 Human Fire Mage; Caethan -- 60 Human Blood Death Knight; Danee -- 41 Human Outlaw Rogue; Ainsleigh -- 52 Dark Iron Dwarf Fury Warrior; Mihena -- 44 Void Elf Affliction Warlock; Chiyan -- 41 Pandaren Brewmaster Monk; Threkk -- 40 Orc Fury Warrior; Alliera -- 41 Night Elf Havoc Demon Hunter;
Darkmoon Faire-EU:
Sieon -- 45 Blood Elf Retribution Paladin; Kuaryo -- 51 Pandaren Brewmaster Monk
Taelas -- 60 Human Protection Warrior; Shaleen -- 52 Human Retribution Paladin; Raethal -- 51 Worgen Guardian Druid; Szar -- 50 Human Fire Mage; Caethan -- 60 Human Blood Death Knight; Danee -- 41 Human Outlaw Rogue; Ainsleigh -- 52 Dark Iron Dwarf Fury Warrior; Mihena -- 44 Void Elf Affliction Warlock; Chiyan -- 41 Pandaren Brewmaster Monk; Threkk -- 40 Orc Fury Warrior; Alliera -- 41 Night Elf Havoc Demon Hunter;
Darkmoon Faire-EU:
Sieon -- 45 Blood Elf Retribution Paladin; Kuaryo -- 51 Pandaren Brewmaster Monk