Updated Jby Stephen LaGioia: Despite its crude retro style - and perhaps even because of it - the Metal Slug series has maintained a certain charm and appeal even after over two decades. With 13 mainline titles in the series, the progression is fascinating to track from simple 2D side-scrolling shooter all the way to its only 3D entry. RELATED: The Best Run & Gun Games Of All TimeĪs the flagship title for SNK, one of the biggest developers of that era, Metal Slug holds an important place in the hearts of gamers everywhere.