"Chroma Incredible Brush Cleaner is a superb soap for cleaning and maintaining all your fine brushes. It cleans and preserves natural or synthetic brushes, easily removes acrylic, oil or lacquer and even removes dried paint that is years old."I can attest that all of the above is true - especially the years-old paint bit. For dried paint, make a slightly dilute solution of soap by adding water, and then coat the brush head. Best results are achieved if you leave the brush for a few hours.
The only other soap I have tried is Da Vinci All Natural Brush Soap with Conditioner (Kernseife). This curd soap is also an excellent cleaner, and seems kinder - not a lot of lather is produced from this bar of soap. The Da Vinci is not effective at removing old paint, but for normal use either cleaner will leave your brushes soft and squeaky clean. :)
After oil painting, my normal cleaning process is as follows: First, I use oil and then odourless solvent to clean my brushes, then I wash them with soap. I swish my brushes in a few drops of the soap cleaner, and then 'comb' out the paint with a soft toothbrush to loosen it. I rinse then repeat this process a couple of times. If you don't use any solvents to clean brushes your brushes first, the Chroma or Da Vinci will still do a brilliant job - it just takes a little longer.
Usually a watercolourist has no need for brush soap, but once a year or so I do like to clean my most used brushes. The heavily staining pigments like phthalocyanine blue/green and dioxazine purple do build up. After cleaning a watercolour brush, make sure the brush is rinsed thoroughly.
Chroma Incredible Brush Cleaner - 250ml (~$12AU)
Da Vinci All Natural Brush Soap - 100g (~$9AU)