There is no single best deck stain or sealer. The best wood protector for Texas may not perform the same in Louisiana. I will attempt to to sort out for you among various options, and give you the power to make the best choice for the care and maintenance of your deck or wood structure.
If you require more in-depth personal consultation for your project, I offer my expertise to you with a scheduled consultation to give you detailed and specific project solutions.
If you would like to do your own research, here are some articles to browse: Deck Care Blog
If a previous product has been applied, you must accept the fact that that product must be effectively stripped and removed, if a different better product will be applied. Otherwise, you should clean as recommended, and recoat with the existing sealer or stain previously used.
Preparing Your Decking Wood Please Read This!