The original that started it all. It introduced the POW rescuing mechanic, the titular Slug tank, and the iconic "Heavy Machine Gun." The ROM is relatively small (approx. 40 MB), but the difficulty is legendary.
Furthermore, the "conversion" scene has become controversial. Many sellers on eBay flash Metal Slug ROMs onto counterfeit cartridges and sell them for $300 as "reproductions." While purists hate this, it keeps the hardware alive. metal slug neo geo roms