Asking for "the most [x]" is a hard question, but I can list some of the weirder ones I know of.
Zanac requires a Famicom Keyboard, that one was fun to test on real hardware. (I wonder where my FK is...)
Mortal Kombat's EJB menu code took years to discover (but is mostly just buttons)
I'm having trouble remembering some beyond stuff like the Taito code, but I'm sure there are others.
