The problem with treachery is that in the areas where your merc will actually get cursed often, ie chaos/baal, it will get cursed often enough that treachery has limited impact, chaos especially, where curses will just get spammed so often to be constantly reset.
A fortitude is easily the highest defense offering item unless you're a barbarian with shout or paladin with defiance, because the global +140% defense on chilling armor multiplies his overall defense.
But defense isn't the only way to keep your merc alive. Fortitude builds can only have 31% DR from ber/ber CoA, a guardian angel setup has 90% all resists but only caps out at 39% DR, while treachery goes up to 46% dr, and shaftstop can go 50% dr and have 1 socket to spare
How useful your mercenary defense is, is kinda debatable. A level 98 holy frost merc (1671 base d) with a perfect ber/ber CoA (399) + 100% perfect eth glitched fortitude lacquered plate (3648) + phoenix polearm would have 15438 defense when chilling armor procs, whereas with a fortitude polearm he'd have 26874 defense. Against your average trash mob blood lord in worldstone keep 3 with 5929 AR and level 82 when he's 98, that could make the CTH formula return a respective 25.3% and 16.5%. Which doesn't apply when he's running, or against autohit attacks, or spells, auras, etc. When nihlithak is spamming corpse explosions, a phoenix weapon is going to keep your merc alive longer than any other weapon, but forfeiting the weapon slot means losing a more valuable aura.
This post was edited by Goomshill on Dec 15 2017 10:06pm