I found, any monster with a 'name' is good. So for example Exo, Corgul ( these with drop one or two per run per char) give or take..
However, if you are farming for no names legendaries i found high end mobs such as exo,corgul do not drop them very often or at all.. For these I found spamming Purifier, Coil.. With a looting script enabled me to get quite a few!