Scramble

Defender may have beat it to market by two months, but Scramble introduced what would be the more lasting model of side-scrolling shooter, as you go through on your fixed scroll ever rightward avoiding terrain and shooting enemies... and fuel (don't ask how it ends up in your tank). The refueling mechanic would be influential for a while, but it's all the rest of it that would give us the Gradiuses of this world, once things were gussied up with nicer graphics and more varied and fun weapons.