For pre-engineered flooring, Aqua Barrier is a good product to use.
For safety and a more "green building" approach, there is also a LEED-compliant product -- Green Resource Underlayment -- a GREAT product.
- Make sure that your installer is familiar with the installation of this product, or maybe print out the PDF I linked to above.
- The product is made from 100% recyclable materials.
For more information:
Read Ian MacLeod's Q&A "I'm concerned about toxic offgassing from OSB subflooring and roofing. Are safer products available?"