When you talk about alien shooters in the 1980's you can not go without talking about Space Invaders. You are at the bottom of the screen with your gun shooting up at rows of aliens in the sky that are trying to come down and defeat you. You have shields that protect you ( while you still have 'em) and as you clear each level things get more and more complicated. Highly addictive will keep you busy for a long time.
This is one of the best selling games in the Atari library and has held up very well to the test of time.