I'd check out http://dropcalc.silospen.com/item.php and play around with it. On 1.13 with players 1 and 300 MF, Baal is the best boss for farming one with 1:15989 odds followed by Nihlithak at 1:121701 odds. But champion mobs in all level 85 zones are 1:110442 odds so those would definitely be the place to be. Clearing ancient tunnels or the pit should be much faster than a Baal run so it would still be faster to farm those spots.