This is a tough one for me, but I'm going to have to go with Rastan. It was so ahead of its time in 1986. I still play it probably at least once a month.
A close personal 2nd would be Contra, and Mortal Kombat II. Eternal classics!
Wizard is about to die....pump in some more quarters!! What a perfect way to suck money out of the kids . You couldn't just die and leave your friend all alone to fend for himself!!