In your opinion and experience, what is the best coin holder for keeping coins safe from corrosion or other environmental changes? Slabs don't count. :' )

I have always liked Air-Tite holders. They are made of clear acrylic plastic. The model I prefer has rings of black or white polyethylene that hold the coin. They are PVC-free and chemically inert and give excellent protection against moisture. I put all of my most important coins in them. Many retailers sell them but here is the manufacturer's website if you want to read more about them.

http://www.airtiteholders.com/index.html