METAL SLUG 2 / SNK / Arcade
Metal Slug 2 is essentially a new levels pack for the first Metal Slug game - the engine is virtually unchanged and many of the sprites are re-used. Somehow, however, it comes out seeming improved.
I chalk that up mostly to a better sense of level layout and design. Aside from more varied backgrounds and enemy types, levels often feature a series of vertical jumps merged with horizontal movement, and there's some cool boss battles that surpass anything from the previous game, like a set of mechanical jaws that chases you up a huge pillar at the end of the second level. The layouts also negate (somewhat) the game's greatest gameplay weakness, which is that you can't really ever fire diagonally.
A couple new characters - both female - join the nameless grunts from the first game, and you can now select which of the four you want at the outset of the game and after burning up each credit. The "Super Vehicle" tanks and the Hobo Soldiers both make a reappearance, but there's a cute twist on the former in the desert levels (you get a camel with a mounted machine gun instead of the normal tank ... also happens to be a little easier to control and aim your fire. Go camels.)
Overall, same story as the first game - looks great, plays great, sounds great, but I felt the gameplay/layout was improved enough here to make a difference and elevate this one a notch over the prequel. There were some issues with slowdown and glitches in the original release, prompting the game to be remade as Metal Slug X, which you may want to check out instead.
* Gameplay Video