what's the best insulation for a walking attic,also the most economic

The answer depends somewhat on your layout. We're about to do our attic and will use fiberglass, but our contractor is also installing styrofoam sections to keep it tight in place while still allowing air to pass through (to prevent condensation). Here's a good article I found that takes you through the various types.
http://www.oldhouseweb.com/stories/Detailed/10169.shtml