Its hard to say what is the "best" DK tanking weapon.
Expertise on a weapon makes it easier to be Expertise capped, so you don't have to gem or enchant for it.
As a general rule, I go with the weapon that has the most Stamina.
If you're already hit and expertise capped, then prioritize weapon base dmg next. And you don't even need to be Hit capped, except on bosses that require constant taunting like Gormok, as I just discovered.
However, keep in mind that even if you're hit/exp capped, a weapon like Mourning Malice (or Sharpened Obsidian Edge Blade) will let you replace your Expertise gems with Stam gems. So sometimes, dropping ilvls to something like Mourning Malice is a valid move.
Also, when you say use HB as an opener, is that without diseases on? =S
For trash packs, without. For bosses, with (your rotation minus Pest). Edit: for bosses, use Oblit instead of HB.
Even without, it's still the strongest opener for Frost. It frontloads more threat more quickly than any other ability. Extremely useful when pugging w/ trigger happy dps who don't wait for the tank to build aggro.
Even with the Glyph of HB, the first HB is diseaseless. But the subsequent BB + DnD are buffed by Glacier Rot and Tundra Stalker, as are all subsequent HBs. That sequence is usually enough to ensure your aggro lead for the rest of the fight, at least on trash.