#3 Metroid – The Everything Passwords
Metroid was one of the first games to use an actual password system, rather than just pressing a series of buttons on the controller; this was a pretty big step for Nintendo games. One such password unlocked all of Samus' weapons and powers in addition to one that unlocked her armor; however, they were virtually impossible to use as they were only meant as a built-in security override for the cartridge. The codes were also literally impossible to find, even if you reverse engineered the whole password system. The system was crackable, but Metroid had a “parity check” that rejected faked entries, which included ones mentioned above.
This is why the NARPAS SWORD (NARPAS SWORD0:000000 000000) wasn't discovered until recently.