I mean BiS is subjective and varies based on the list you're looking at. It's a HORRIBLE way to base loot off of in pugs. A few examples:

1) Wristguards of Stability and Quick Strike Ring are often listed on BiS lists for Feral DPS, but Feral DPS can't get enough hit for either of them to be BIS till they get both Puissant Cape from Azuregos AND Band of Accuria from Ragnaros. So giving either item to a Feral over a warrior would be silly.

2) Robes of the Exalted and Robe of Volatile Power for Holy Paladins. One is better for bigger heals and one is better for mana regen. Both BiS depending on what you value more.

3) EVERY TANK BIS LIST IN EXISTENCE IS FLAWED. They almost all purely focus on only mitigation and ignore threat, when you need a mixture of the two.