So you've just gotten a shiny new weapon. Of course, like any good Tankadin, you want to get a weapon enchant on that bad boy.
Let's take a look at your options. We'll discount cosmetic factors like the glow effect. Tanks aren't looking for style points, they want to survive. Now you can can survive, in style, but you are probably going to be a Dwarf.
Accuracy - Increase crit and hit rating by 25.
Blade Ward - Chance to increase parry(up to about 2%) and damage on parry as well, up to 5 stacks. I think.
Blood Draining - Leech health from target and chance to give about 2000 health when below 35%.
Exceptional Agility - Increase agility by 26
Mongoose - Chance to get 120 Agi (about 1.2% dodge and 1.8% crit( and increase weapon speed.
One of these things is not like the other
If you want your weapon enchant to enhance your threat generation, you've made your job of selecting an enchant much easier.
Accuracy is hands down the best threat enchant for a Tankadin. This is ESPECIALLY true as you begin to aquire Tier 9 level gear and you watch your Hit Rating plummet to nearly zero and your Expertise shoot through the roof to near the soft caps.
Note that if you are bit cash strapped, you can sub in a Titanium Weapon Chain instead of Accuracy.
Roll the dice
All survivabilty enchants are proc based. If you are looking for a static 'always on' type of enchant, look no further than 26 Agility.
Now when you get down to the three proc based enchants, you have 2 avoidance (Blade Ward and Mongoose) and one effective health enchant (Blood Draining).
My preference is Blood Draining. It's like having a Health Pot that you drink at the exact moment you need one the most, and unlike a potion, you can have it multiple times during a fight. It does take a little time to get a full stack up, but generally it's going to be there when you need it. Early on, there was some concern over its interaction with Ardent Defender, but with the new Ardent Defender, there is no longer any reason for concern.
Blade Ward has become more popular in 3.2 than it was previously, thanks to a buffed proc rate. However, the other issues with Blade Ward remain. It can be consumed by regular parries (i.e. those caused by other parry rating) and thus gets worse as your gear gets better. The proc actually provides less threat than Potency and no benefit to taunts that accuracy or a weapon chain would.
Mongoose remains a popular choice. I think part of the reason is that Tankadins wanted to use this during it's heyday of Burning Crusade, but we generally had to use Spell Power enchants on our weapons to be competitive with Warriors and Druids on threat. 'Goose will proc less for a Level 80 character than it would for a Level 70, and due to level scaling, 120 Agility gives less dodge and armor to a Level 80 than it did to a Level 70. It's still a decent enchant, and if you love to roll the dice, it might be fun.
Bottom line it for me
I might consider Accuracy, Blade Ward or Mongoose for my Daily Heroics weapon, but I'll want Blood Draining on my Boss Tanking weapon.