Greater Dodge

With the addition of this enchant, options for tanks are as follows:

http://www.wowhead.com/?spell=47051
Enchant Cloak - Steelweave = 12 defense rating/5 defense skill (0.2% miss, dodge, block, and parry for a total of 0.6% pure avoidance plus 0.2% chance to block)

http://www.wowhead.com/?spell=34004
Enchant Cloak - Greater Agility = 12 agility (0.36% crit, 0.4% dodge, and 24 armor)

http://www.wowhead.com/?spell=25086
Enchant Cloak - Dodge = 12 dodge rating (0.63% dodge)

http://www.wowhead.com/?spell=27961
Enchant Cloak - Major Armor = 120 armor (0.15% melee damage reduction)

Greater Agility still appears to be the best choice for a mixture of survivability and threat generation, but Steelweave most likely beats Dodge for the best defensive option.

Kaziel, armor does not suffer from diminishing returns. You need more armor to get progressively higher reduction percentages, but the higher the percentages get the more a percentage point is worth. Eg. going from 60% reduction to 61% reduction is a bigger improvement than going from 55% reduction to 56% reduction. The armor formula is designed so that armor is equally good at any point until you reach the cap of 75% damage reduction. That is only ever an issue for druids. @Hektor: The agility enchant is not good for warrior tanks. They should go with dodge or defense rating (which are more

Another bonus with this enchant is that it gives you more flexibility when you are wearning alternate gear, eg resistance gear for specific encounters, and need to juggle equipment to stay at or above the defense threshhold (eg 490 for warriors).