If you're looking for a side scrolling beat em up similar to Final Fight and Streets of Rage then Undercover Cops is the game you're looking for. It's classic 2D button mashing at it's best with graphics and sounds that look and sound like just like the arcade game that this was ported from.
The first thing that one notices about Undercover Cops is that the art style is very similar to the Metal Slug series. Many of the people who made Undercover Cops later started Nazca Corporation which was responsible for Metal Slug.
The US saw a very small release of the arcade game and a SNES release of the port was planned and even reviewed. Unfortunately the game was cancelled before it hit stores. A Japanese port was released and the game is completely playable with only the introductory screens being in Japanese.
Undercover Cops is a great game in a genre that has been long forgotten but is now making a slow comeback through downloadable games services. Give it a try!