Best tank weapon enchant for paladin for the hardest boss of this expansion, LK 25 heroic. Other bosses don't really matter. And please state your reason in detail would be really helpful. Also would be really helpful if you know if the enchants procs are bugged or working 100% intended.
- Reliable, will always proc when you're under 35% while both Blade Ward and Mongoose may or may not be there when you need them most
- Buys you some extra EHP at a point where you need it most. The amount may not look like much at 5 stacks, but it is a considerable improvement to mongoose if you just compare the flat HP to the agility proc.
A few words on the other two:
- Blade Ward is purely threat oriented. The extra avoid it provides is very small and should get consumed upon your next parry, according to wowhead comments, which drastically reduce its effectiveness as an avoid enchant.
- Mongoose is both offensive and defensive. It'll give you a healthy amount of armor, some small amount of dodge and a bit of crit. It's a pretty decent mixture and I'd recommend it over Blade Ward.
To avoid senseless arguing, what I've said above is an opinion from the PoV of someone who does end-game raiding with decently/well competent groups. Some would agree with me, others won't ¯\_(ツ)_/¯.