Chopper Command

A minor Classic in my Opinion.
Fast and furious - but lacking variety. I, personally, play it often but not for long times...
A better Defender-type Game is STARGATE (aka Defender II) for the Atari 2600 or SUBTERRANEA, which is strangely widely unknown/underrated, and my Favourite for that Type of Games!

I remember it being a choice between this and 2600 Defender for my birthday one year. This was the winner and what a great game it is. This is a pure Activison classic.