Space Fever II
The third best game ever made for the original Game Boy. It is a sequel to the original game "Space Fever", which was basically a rip-off of Space Invaders except in color. Nintendo listened to the minor complaints regarding the first game and made this.
It is actually closer to Galaga or 1943, in that the game scrolls vertically. There are two kinds of shots, single and double. There are three bosses: a weird fat guy's severed head, the severed head of Mario's inbred grandson, and the Game Face (just the second boss again if there's not one).
The game is so awesome it was never released on its own in the U.S., Japan, or even the Netherlands. It was, however, included on the Game Boy Camera.
PRO TIP: Don't press the B button, ever. Your ship will self-destruct and your onboard computer will die of cancer of the silicon.