What should I use to insulate my attic?
You wouldn't put a heating duct outside your house, so why would you put one in a vented attic?

Since your furnace and ducts are in your attic -- I see only one sane option here, and that's to insulate the roof, forming an unvented, conditioned attic.  Don't skimp on the insulation here -- I would use lots of R-value, R-50 at a minimum, to keep any snow that may get on the roof from melting and refreezing into an ice dam.

In general, this is most often done with open cell spray foam applied to the underside of the deck, but this can also be done by applying at least two layers of foamboard with the joints staggered in both directions to the roof deck and then putting an overdeck over the top of the foamboard, creating an assembly much like a commercial compact roof, as in the illustration below (from the Building Science folks).



Furthermore, I would make sure that there's air exchange between the attic and the rest of the system, so the attic doesn't turn into a hot, wet, condensing mess during the summer.

See BSI-046: Dam Ice Dam and BSI-077: Cool Hand Luke Meets Attics for more details.