Wings of Death.
Wings of Death is perhaps one of the definitive vertically-scrolling shoot'em ups for the Atari ST, a yardstick by which other such games are measured. This game has everything - hordes of monsters, oodles of powerups, strange and unexpected phenomena, smooth scrolling, extremely slick graphics, catchy music, big and attractive bosses - it's hard to find a fault, no matter how closely you look. It's even enhanced for the STe, and supports blitter and STe DMA stereo sound effects.
You play one of a variety of creatures (you can mutate between the different types of creature by collecting various powerups) through several levels of vertically scrolling mayhem. The only creature I can recognize as such is a dragon, who spurts quite satisfying blasts of fire. The other weapons available are triangles, strange spheres which split into several shots when they reach the middle of the screen, and a sort of windscreen-wiper weapon.
Verdict: If you don't have this game, you have missed out on hours of joystick-wrenching fun. They don't come any better than Wings of Death.