Basic Idea: Gradius, with better graphics and less dying.
Review: My favorite shooter of all-time, Life Force is basically perfect. It can be played alone or with a friend. The power-ups kick ass. The backgrounds are creative and visually stunning. The music fits the mood. The boss battles, while relatively easy, are great fun. And the final level, frantically trying to escape the alien your plane has just destroyed, is really intense.
Granted, I’ve never won this game without the Konami Code. But I have gotten pretty far without it, and there’s only a couple times where the game feels unfair at all. If you only play one shooter on the NES, it has to be Life Force.