I finally unlocked this game by playing Alien Storm with my wife.  Everything's better with co-op.  Alien Syndrome an arcade game from 1987.  It's not unlike a sci-fi version of gauntlet.  It's a top down shooter where your goal is to rescue a number of hostages before the timer expires, then head for the level exit.   You then face a boss battle before moving on to the next level.  The game is quite unforgiving as you die instantly from one hit.  The graphics are very appealing for it's era, and the action is generally satisfying.  Co-op play as always makes the game better.  (Only two player co-op, however).  I value this game initially at 50 cents.  Like so many it's replayability is limited by it's difficulty.  You can continue, but there appeared to be a limit.   Perhaps it was just that both players couldn't die at the same time.

I also unlocked and played Fantasy Zone, but I didn't play enough say much on it yet.  It struck me as a psychedelic cross between Scramble and Chopper Command.

I've now unlocked everything except Zaxxon.  I'd really like to get that one, but doing so requires me to play more DeCap Attack, which just doesn't interest me right now.   Maybe I can just look up what I need to do on a FAQ, if such things exist for DeCap attack.