If you hate DRM, this is your best option.
If you are debating between regions, the USA release of Metal Slug XX has the most balanced localization. The Japanese version keeps difficult arcade-style continues (limited to 4 per credit), while the USA version allows unlimited continues with a slight score penalty. Additionally, the English voice lines for the POWs ("Thank you!") are intentionally cheesy and beloved by fans. Download Metal Slug XX -USA-
Available for purchase on Steam, supporting 4K resolutions and including Leona Heidern as a free playable character from the start. If you hate DRM, this is your best option