I think the answer to that question depends on what soap and how your body (intestines) tolerate it.
Many people have mentioned castile soap. Yes, understandably you need to pay attention as to what soap and ingredients you use in your enema recipe solutions as not all ingredients are practically safe, or at the very least well tolerated.
I have spoken in one of my previous posts about osmosis (boring high school chemistry) and how substances can be absorbed via the intestinal membrane and enter your bloodstream. Feel free to research it yourself, but even alcoholic beverages that are safe to be consumed orally in moderation become extremely toxic when injected inside your intestines. There has been documented deaths from alcohol poisoning because people have consumed alcoholic beverages via an enema - DO NOT do it!
However, I will have faith in humanity and trust that all of us here on Zity who have enema passions to varying degrees, are good wise well-adjusted individuals who will never perform anything dangerous onto ourselves and others.
As such, going by the response others have posted to this question, I would say that my specific answer is that it depends on what works well as an individual for your intestines and how they tolerate it - safely.
Many have mentioned some kind of castile soap. Unfortunately, here in Australia castile soap is perhaps not as common as in the USA. Yes, what is labelled as castile soap is available in a few shops, and I can purchase it (usually from the USA) on the internet – (very expensive + postage).
There are also some ‘natural organic’ shops that sell many foods and products like goat’s milk and olive oil soaps that are labelled natural, safe, and free from the nasty usual mass-produced soap ingredients. These are the types I use for my enema recipe.
So, after boiling some filtered water, I dissolve 1 teaspoon of table salt per 1 litre of water in a suitable jug, which I use as the base solution because I desire the water to be the same salinity as my body to stop the osmosis process, I mentioned above, and thus keep the same amount of water inside my intestine during my enema. At this point I am only interested in dissolving the salt, so I am not concerned about the boiling water temperature because I will be sure to test and adjust it by adding cold water in the jug. However, whilst the water is cooler but still far too hot and unsafe for my enema, I transfer it to my suspended 1-gallon silicone enema bag and just let it stand inside for a while until the safe temperature is reached. It is also during this time while the still-too-hot water is standing inside the enema bag that I proceed to dangle a bar of the natural soap tied to a shoestring inside, just like a teabag. Usually, I leave the soap submerged in the hottish salty water for a while to let the soap infuse further as it dissolves thanks to the heat. For me, I only want to dissolve enough soap to make the water somewhat hazy, not opaque. I don't want suds like a bubble bath.
By the time the water temperature has adequately cooled, the salty water is also soapy enough for me to self-administer my enema safely with a soft 30Fr, 50cm rubber colon tube that I insert up into my descending colon, and just let that warm soapy water trickle down into my rectum as I fill up.
(I think I will go and have one now.)
Goodbye for now.