The best would be distilled water. Chlorine and fluoride free boiled water would be next best. Boiled water is a poor substitute IMO. Putting chloride into the colon might have an effect on colonic flora.
Filtered water is not always as useful as it might sound, it depends upon what is filtered out and many cheap filters are not much use IMO. Mine removes chloride and fluoride but I have some doubts about it now because it has silver with the idea that it is antibacterial but bacteria are resistant to silver so it is not as much use as it once was. Reverse Osmosis is very good but the flow rate is very low.