@JayJ yeah, I'd mostly agree with that. Metal Slug and Samurai Shodown do seem to follow similar paths. I would say both of those properties experienced a dip and then a bit of a rise before falling off again. I agree that Slug 2 and X are probably the best. I prefer X for its technical improvements. I completely agree about Slug 3. Its massive difficulty spike and long final stage hurt it for me. To my surprise, I enjoyed Slug 5 a lot. The difficulty is much more manageable like the games prior to part three, and it has other things going for it. I understand if people didn't like the slide move or other things, but that game worked for me at least. I don't know if you'd agree, but I do consider it a nice improvement over Slug 4.

Same deal with Samurai Shodown. Part three had some nice sprite work, but was lacking in gameplay. I did think part four was much better and a good game as well, even if it may have lacked the magic of part two for many. I didn't like what followed after, though.