I am not well versed in these games, but I am aware there are Metal Slug 4 and 5 as well? were they made by other house? Were those inferior graphically and gameplay wise?
MS4 was produced during their bankruptcy (like KoF 2001) and it's a mess visually and really unbalanced gameplay-wise. Significantly worse than the previous games.
I actually really like Metal Slug 5's aesthetics, soundtrack and gameplay additions (A transformable Tank/Robot Slug! All new grunt enemies with a cool special ops theme! And tribal enemies! And cultists!). In some ways it's my favorite game in the series... but the game is
clearly incomplete. It's the shortest Metal Slug and doesn't even have a proper ending. The worst part is that everything introduced by this game was pretty much thrown away afterwards, rather than building up on it.
For Metal Slug 6, they moved to new hardware, resulting in more detailed backgrounds, but it's missing a lot of the variety seen in the first 5 games, and also lacks huge moving boss sprites (maybe for cost reasons),background animations and the enemy variety from 3 or 5, in spite of mostly reusing enemies from Metal Slug 1 to 3.