Moisture-resistant MDF is the most popular choice for skirting boards in the UK. It machines to a smooth, consistent finish without the knots or grain variations found in natural wood, making it ideal for painting. MDF won't warp or twist over time and works well in kitchens, bathrooms and other areas where humidity fluctuates.
Softwood (typically pine) offers a more affordable alternative and suits those who prefer working with natural timber. For a visible wood grain finish rather than paint, hardwoods like oak or tulipwood provide a premium look, though at a higher price point.