N.B. This is for idle play and refers only to obtaining the best CpS boost, it is not concerned with decrease in number of golden cookies or the like.
In my opinion, based on tinkering with my spreadsheet and seeing the resultant change in CpS, the best arrangement is:
Diamond slot: Mokalsium
Ruby slot: Holobore
Jade slot: Jeremy
This gives a 30% improvement over the arrangement I have at present, which is Holobore/Jeremy/Mokalsium. Hence I intend to switch them around as soon as I have enough lumps.
It's possible the best arrangement may depend on what you own, specifically on how much milk and how many kittens. Obviously the bigger the boost you are getting from milk, the more attractive this makes the Mokalsium spirit. It may be that in the early stages of the game a different arrangement may be better. Some time when I have a dull moment I'll play with the numbers and report back.
Also in the beta version, note that Jeremy in the Ruby slot gives a 5% boost, NOT 6% as stated by the documentation. I am now 99.99% sure of this. Moreover the multiplier, as displayed on the Stats screen, does not include this 5%, though it does include the 15% from Holobore. I have not yet been able to check whether this is fixed in the new live version.