According to the search results, there are different opinions on what the purest form of water is. Here are the different options:

• Rainwater: Rainwater is considered the purest form of water by some sources. Rainwater is sourced from the clouds and never touches the ground, which makes it free from minerals and salts. However, other sources argue that rainwater is not necessarily pure, as it brings down impurities and particles present in the atmosphere along with it.

• Distilled water: Distilled water is considered the purest form of water by some sources. Distilled water is produced by vaporizing water and then condensing the steam back into liquid form, which removes impurities and minerals.

• Filtered water: Filtered water is another option for pure water. It is source water that has been purified of all pollutants.

In conclusion, the purest form of water depends on the definition of "pure" and the method of purification. Rainwater, distilled water, and filtered water are all considered pure by different sources.