What is the best watercolour paper for stretching on a flat board?

Acid-free paper is the best paper to use if you want your work to last a long time. This is because acidity embrittles the paper and eventually makes it too weak to handle.

Whatever paper you choose, use the heaviest sheet you can afford, for the simple reason that it will be more robust and less likely to bend in long-term storage. Paper can be stretched up to a size of approximately 1.5m x 2.5m but if you go beyond that, the tension in the paper will be too strong.