Honestly this game is just overall pretty fun for an arcade game, the visuals are pretty great, it has a gritty and dark but also endearing artstyle full of exagerations and the animation is gorgeously well done, it feels smooth and everything has so much character and personality

Metal Slug 1 lays the groundwork for the next games, it's probably the most simple of the series, it does nothing crazy or out of the ordinary like the rest, and for some it might be the reason why they prefer it, a simple military style shoot em up sidescroller that can be played for a good 30 or 40 minutes, in this game in particular there's not much variation on what you can do in the levels so the replayability of it drops in comparison, but as an experience it's pretty good

The controls are simple, but so, so much better than other arcade games (The simpsons arcade and some other beat em ups) that have only two buttons with no real depth, this one has the astonishing amount of 3 buttons, jump, shoot, and grenade, the thing is, the amount does not quite matter that much, but the controls are simply better as the variety in the combat is a lot higher, when you are close to an enemy you automatically stab them melee, when you are far away you can shoot in the direction of your analog stick, and you get a wide variety of weapons and vehicles throughout the level and exploration that is enough to keep the game feeling fresh in comparison to other games, it's enough to make an arcade game not feel like it's a cashgrab with no depth whatsoever.

Also all the metal slug music slaps, they had no reason to make it that good but it's really great