Setup used was: Reaper's Toll (Ber) and Guillaume's 40/15
I did it with a level 90 merc, with Might Aura level 18 and Fanaticism Aura level 9 (party). This gives a base enhanced damage of 603% without the armor.

Treachery: 603% ED, 5 attacks/sec, 55% CB, 48% DS, 0% OW
Duress: 623% ED, 4.16 attacks/sec, 70% CB, 48% DS, 33% OW
Black Hades: 683% ED, 5 attacks/sec, 55% CB, 48% DS, 0% OW
Fortitude: 903% ED, 4.16 attacks/sec, 55% CB, 48% DS, 0% OW

I did not add the increased damage to Demons from Black Hades, because I wanted to compare in case the target was not a Demon. Still, I chose Diablo as a target in a 1-player game as the target.
Here are the times to kill Diablo, just with the mercenary. Diablo has a base damage reduction of 45%, but Decrepify brings it down to -5%.

Treachery: 33 hits, 6.60 seconds
Duress: 29 hits, 6.97 seconds
Black Hades: 31 hits, 6.20 seconds
Fortitude: 28 hits, 6.73 seconds

All times are close, but Black Hades is the fastest.

If you don't need the Fade effect from Treachery to keep your mercenary alive, Black Hades seems like a good choice