The Underdog Evolution: A Retrospective on Metal Slug 6 (MAME Atomiswave)
Metal Slug 6 is the "forgotten good one." It sits awkwardly between the Neo-Geo classics and the later Metal Slug 7 (DS) / XX . For MAME users, it is absolutely worth the extra BIOS hassle. The character variety and weapon-stock system modernize the formula without ruining the pixel-perfect run-and-gun feel.
Note on legality: The Metal Slug 6 ROM is copyright SNK Corporation. This review is for educational/archival discussion of an out-of-print arcade board (Atomiswave). Always dump your own PCBs.