Quarter Pounder with Cheese (or 2, if you are hungry), no bun (I just take the top off, and kind of slide it off the bottom), no ketchup if you are very strict; however, I find the ketchup minimal, usually sticking to the bun, as does, unfortunately, the cheese sometimes.

I have done the Bic Mac with no bun, but I find it somewhat unsatisfying; those little burgers are, well, little. My memories of Big Macs when I was a kid include stuffing fries in each layer; somehow the bunless, fryless Big Mac no longer seems as big or as good as it once did.