The basic option for a crawl space vapor barrier is a simple 6 Mil black plastic vapor barrier. 6 Mil is the minimum mil thickness that is accepted per building codes for ground space coverage. However, this is only the minimum. This will need to be replaced every few years to keep it working properly as it is prone to shifting out of place, rips, tears, and is easy for critters to chew through.
For this reason we like to use a 20 Mil woven vapor barrier called CleanSpace when we do a full encapsulation. This material comes with a 25 year warranty against rips and tears, and its woven fiber deters critters from chewing through it. This material also is white on the side that will be visible, making it easy to check over for general maintenance and brightens the space. This material is used during a full encapsulation, meaning it will be installed under every wooden post, anchored next to the rim joists and sealed fully into place with a special four inch wide waterproof tape to keep it fully in place.