According to Consumer Search, the top pick from professional deck builders and paint professionals is Sikkens Cetol SRD Semi-Transparent Stain. This particular stain will penetrate the wood, preventing flaking and peeling of the stain.
Paint professionals recommend the higher-priced items simply for their durability and high quality. Additionally, water-based stains are a better choice than oil, and so is using synthetic resins instead of natural ones, which can become a breeding ground for mold and algae.
Semitransparent stain tends to be the most highly recommended among choices of clear, semi and solid. The clear and solid stains do not hold up to UV rays as well as semitransparent. In addition, semitransparent stains are tinted, thus allowing you to see the grain of wood through the tint.