ASTRO WARRIOR / Sega / Sega Master System
From the screenshots this one really doesn't look like much, but I was pleasantly surprised to find it was a decent old shooter. Definitely on the basic and primitive side, but the pace of it and the enemy variety and layouts are pretty good.
It's a 1986 shooter from Sega and it shows ... as you can see the backgrounds are nothing but flat empty black space, and all the sprites are very simplistic. The gameplay is pretty solid, though. The game is on the easy side but throws you for a loop here and there with stuff like the unexpected Barrage of Deadly Corn Nuts at the beginning of the second level. Also, bosses tend to go into Rage Mode when you destroy all their little appendages and the game briefly turns into "bullet hell" until you can land the final shots on them.
Unfortunately, like most old SMS action games, there's no continues whatsoever, so get ready for frustration as you get kicked back to the title screen every time you exhaust your supply of three lives.
* Gameplay Video