My favorite game all time is Shinobi. I used to play it in the grocery store when I was barely tall enough to reach the joysticks and it's always been a game I can jump into and play any time. The first boss scared the crap out of me the first time I saw Ken Oh, that shit was wicked. Shinobi and Rolling Thunder were both my favorites but Shinobi wins by a small margin, and it's sequel Shadow Dancer was great too.