Namco's answer to Space Invaders, Galaxian is pretty great. I love playing old shooters because the premise is so simple and there's a wide variety of methods that developers used to riff on the same ideas. Galaxian relies on different AI of enemy troops (Galaxians) that do the usual steps to the left and back to the right, but never down.

The reason they don't gradually move down is that Galaxians will suddenly dive bomb at you, take a few shots and then rejoin the formation. Meaning you're simultaneously gunning down Galaxians, evading dive bombers taking fire and shooting in response and taking stock of other aliens that could dive bomb at you at any second. Bonus points are accrued based on colour, whether the Galaxian is in formation or diving, with the flagship being worth most dependant on whether it is dive bombing with or without escorts.

I sort of love it, much more unpredictable than Space Invaders.