what defines a tanking weapon?

so, my DK is only level 56, but if I ever complete the starting zone (for the 100th time), my plan was to try tanking my way from 58-85.

in terms of armor, it seems easy enough, but I was a little curious -- how do DK's chose their tanking weapon? just whatever 2H sword/axe/mace has the most dps?
Before 80, [edit: the one with the highest end-damage (max damage).] After 80, the one with mastery.

edit: Woops, forgot that swing-speeds aren't standardized sub-85.
Euliat is right. Higher DPS sub-80. If you get one that has dodge/parry one it, awesome, don't stress yourself over it. After you learn your mastery, any 2h will work, but you would prefer one with mastery over one without.