From Diablo Wiki
Best in Slot = BiS.
This acronym is used to describe the best possible piece of equipment for each "slot" in the paperdoll. Which item is the best often often varies between classes and even between builds of the same class, and seldom is there one certain answer. For instance, a given legendary like Vile Ward (db) may be the best item choice most of the time, but a great roll on a rare shoulders can still beat it for most builds. As such, "BiS" designations are generally meant as guidelines and not absolute rules.
Most of the clear BiS items in Diablo 3 are legendaries that add item properties that rares and other Legendaries can't match in that slot. For instance, the Mempo of Twilight (db) helm has Attack Speed as an inherent property; something that no rare helm can roll with. Mempo of Twilight has several other excellent mods on top of that, but they can be matched by a Rare. Thus it's possible for an exceptionally good Rare helm to be more useful than a poor roll on Mempo, but a great roll on a Mempo will outdo any Rare helm, thus Mempo is the BiS for virtually all classes and builds, at least potentially.
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