What do you use?

[Zookie]

I like to use my Manix 2 with m4 steel, slices through cardboard like a light-saber!

Also my Spear-point ground XM18 has been breaking cardboard down, I love it's thin grind!

 

[Fred S2]

Krein regrind ZDP E4. Watch out for staples!

 

[Fancier]

My Spyderco Mule Cruwear breaks down cardboard every day.