As of this very moment, Riplimb's Lost Collar(hc) is BiS.
However, the rep belt SHOULD be upgradable with a crystallized firestone. Blizzard acknowledged this problem and said they'd be addressing it later. When exactly, if ever, that is, your guess is as good as mine. But once this fix is applied and you're able to obtain a heroic version of the rep belt, that will be BiS.