Maybe it's just me, but I just played Metal Slug 4 & 5 and I'm really not receptive to the Metal Slug games as I once was. The first game of the lineage, which holds the name of series, was so thoroughly detailed and full of action and humor that it blew me away. Metal Slug 2 (and its rehashed version X) came along with a different story, but it kept the same winning formula from the first.
Metal Slug
Then comes Metal Slug 3. Despite taking a very sharp turn to the world of mutated sand crabs, zombies, yetis, and gatling guns strapped to animals, 3 was pretty damn sweet. I'm more geared to only having conventional enemies such as the standard grunts and their military hardware (likes tanks, helis, planes, and transports), rather than huge eels on leashes and bone-wielding sasquatches. But I think 3 and X are the two best in the saga.
Metal Slug 3
Then 4 is conceived... just without SNK. Compared to the others, it's hollow and without the detail or gameplay. Same goes for 5. There's a lot of new ideas in both, but the games fall short.
I have yet to play 6 but I hope it's worthy of the Slug name.