Honestly, out of all the codes I ever used, it had to be the machine gun rocket launcher game genie code for Resident Evil 2. I think it was when you entered the code for rapid-fire, the sub-machine gun, the rocket launcher, and infinite ammo - or at the very least, the rocket launcher and the sub-machine gun. I laughed all the way through the game after I put that one in!
Heh, I remember playing Mortal Kombat at the arcade and just about breaking my neck trying to read the yellowed little scrap of paper with all the fatalities and moves on it taped up in the corner of the screen. I didn't play the game enough to memorise them.