What's the best asset class for the next 1-5 years if we become deflationary?

You're joking, right? If not, you're totally delusional.

 

fed stats show that real interest rates are negative, which is indeed deflationary:

http://delong.typepad.com/sdj/2008/03/real-interest-r.html

looking at history, the best assets in deflation are supposedly near-dated notes of fiscally-responsible governments, but you'll have to wait for mr. market to tell you which currencies are best.