Best vapor barrier material for crawl space encapsulation?

I've seen folks get really worried about this and end up spending way more than they need to. Six-mil polyethylene sheeting is the standard choice for crawl spaces - it's cheap, holds up decently, and does the job most people are looking for. If you're in a really damp area or want something that lasts longer, thick polyethylene or reinforced plastic barriers work better, but honestly the six-mil stuff is what most people go with and it works fine if you seal the seams properly with tape.