The horadric caches can drop probably any legendary ingame, but they also can drop few legendaries only obtainable from cache. Every act except act IV have its specific few legendaries and act IV can drop any legendary from act I-V.
I marked some legendaries with how usefull they are. It is only my opinion but could help you figure out which acts are good to focus farming.
Legendaries only from Act I bounty cache:
Ring of Royal Grandeur +++
Ring of Royal Grandeur - Game Guide - Diablo III
Sanguinary Vambraces +
Sanguinary Vambraces - Game Guide - Diablo III
Pauldrons of the Skeleton King ++
Pauldrons of the Skeleton King - Game Guide - Diablo III
Mad Monarch's Scepter ++
Mad Monarch's Scepter - Game Guide - Diablo III
Golden Gorget of Leoric +++ (My friend got this and said skelets hit for 2m on his wizz)
Golden Gorget of Leoric - Game Guide - Diablo III
Legendaries only from Act II bounty cache:
Ilusory Boots +++
Illusory Boots - Game Guide - Diablo III
Gloves of Worship ++
Gloves of Worship - Game Guide - Diablo III
Cloak of Deception +
Cloak of Deception - Game Guide - Diablo III
Coven's Criterion ++
Coven?s Criterion - Game Guide - Diablo III
Legendaries only from Act III bounty cache:
Avarice Band ++
Avarice Band - Game Guide - Diablo III
Boots of Disregard ++
Boots of Disregard - Game Guide - Diablo III
Burst of Wrath +
Burst of Wrath - Game Guide - Diablo III
Envious Blade +
Envious Blade - Game Guide - Diablo III
Insatiable Belt ++
Insatiable Belt - Game Guide - Diablo III
Pride's Fall +++
Pride's Fall - Game Guide - Diablo III
Legendaries only from Act V:
Death's Bargain +
Death's Bargain - Game Guide - Diablo III
Helltrapper ++
Helltrapper - Game Guide - Diablo III
Pandemonium Loop --- (I hate this ring cos everyone hates fear it ruins party dps hard)
Pandemonium Loop - Game Guide - Diablo III
Soulsmasher +
Soulsmasher - Game Guide - Diablo III
Salvation +++
Salvation - Game Guide - Diablo III
All feedback is welcomed.