Robocop is my most favorite shooter for the Gameboy. This game features 10 unique and challenging levels that aren't too repetitive like many shooters of its era. There are even several puzzle levels which give you a break from continuously mashing the B button. The gameplay is very similar to Robocop Versus the Terminator, but surprisingly, I find the Gameboy version a lot more entertaining. The music is AMAZING. I mean amazing as in you can actually listen to it by itself like a regular song and be entertained. Chiptune listeners should definitely check this game out. The only flaw with this game is the weapon "power ups" which honestly do not give you any advantage, contrary to Contra. It's actually even harder to hit enemies with the shotgun. Overall, this game is pretty impressive.
Here is a video of someone doing a longplay of the game: