Dense-packed cellulose. It is natural, recycled, no toxins, fire resistant, and can absorb a small amount of water.
For more information:
Read Kati Curtis's Q&A "I need to replace insulation. I'm chemically sensitive and need product suggestions."
Also, read GreenHomeGuide's backgrounder "Choosing the Best Insulation Delivers Energy Savings"for comparisons of cellulose, fiberglass, foam, etc.