There are a few factors to consider when choosing the best pipe for an underground water line. The first is the material of the pipe. There are many options including PVC, copper, and iron. Each has its own benefits and drawbacks that need to be considered. For example, PVC is more resistant to corrosion but is not as strong as iron.
The second factor is the diameter of the pipe. The larger the diameter, the more water it can carry. However, a larger pipe will also be more expensive and more difficult to install.
The third factor is the climate in your area. If you live in an area with freezing temperatures, you'll need a pipe that can withstand freezing without breaking. PVC pipes are good for this because they're flexible and can expand slightly when frozen without cracking.
Once you've considered all of these factors, you can choose the best pipe for your underground water line.