Executioner Enchant vs. Mongoose vs. ?? (Haven't theorycrafted yet)
Executioner is the incoming new enchant; it will occasionally ignore 840 Armor on a target.
My hunch -- given history with [item]Bonereaver's Edge[/item] and [item]The Night Blade[/item], is that this is going to be the most reliable proc-based enchant for a tank. The PPM will (from what I've read) be similar to Mongoose or Crusader.
While Mongoose certainly isn't bad (I can't wait to try it!), the Haste is not as beneficial to a tank as it is to anyone else; also, the Crit is similar to proccing a proc; it isn't reliable in the short term.
Executioner, on the other hand, reliably increases Shield Slam damage whenever it procs. This is why [item]The Night Blade[/item] was so powerful and why I replaced my [item]Thunderfury, Blessed Blade of the Windseeker[/item] with it so quickly. Given the number of people who used it for the same reason, it was probably the reason for the nerf on that item, too. :( Few enchants can boast an increase to Shield Slam.
Further, Executioner becomes significantly more powerful with Sunder Armor, since Armor is a curved damage reduction formula. Couple in a couple debuffs and and this shoots up. I'm tempted to try the dagger out again with the enchant, even with it's nerfed proc rate, to see how it works out on raid mobs.
Anyway, no numbers to show, no hard data. If anyone wants to put some together, feel free.