Are there any games or game series that you can't get enough of and seem to have endless depth? I have always been a huge Metal Slug fan and to this day the older games still feel fresh and modern.
The series has a very good sense of humor that I can't get enough of. For example, you have slow and cumbersome vehicles against you that look impressive, but actually don't do a whole lot. There is a tank that is very tall and boasts a huge cannon, but all it can do is drop mines that slowly roll towards you. Some might also remember the giant plane at the start of MS2. It is slow, huge and impressive, but it doesn't do a whole lot except have the tanks it's carrying shoot at you. As an intentional attack or not, the tanks can actually fall on you. The main bad guy, Morden, is a cross between Saddam Hussein and Hitler. Hitler was a huge fan of making big, powerful war machines, but the war machines he wanted weren't always the most efficient in combat, or to make. I don't know if I am stretching it a bit when I say that the developers knew this and made this as part of their theme. Considering the breadth and width of all the references they make to movies, games and history, it wouldn't surprise me. If this wasn't enough, there are all sorts of things you have to fight including mutants, zombies, aliens, robots and whatever else, all done in relatively good taste. They are silly enough to look at, but they can be extremely tough to fight, which lends the game a sense of wacky humor which is grounded in some seriousness.
The series has also fascinated me because of how loud and busy the animations and environments are. The backgrounds are extremely detailed and are filled with all sorts of movie and game references, including one for their earlier submarine themed side-scroller game In the Hunt, and most of them are pretty subtle. The player characters have all sorts of idle animations, death animations, victory animations, even animations for when you are dangerously close to the edge of a platform. Even more impressive is how the grunts you fight against are very versatile in their animations. If they don't see you, they sit around lounging or laughing at each other's jokes, but if they see you, they pause to have a startle animation and then start fighting you. If they kill you, they start laughing. If you respawn, grunts near you are automatically startled. They have emotions ranging from fear to confidence to happiness, which seems an awfully complicated range of emotions for a simple side-scrolling shoot-em-up.
The grunts can die in all sorts of horrific ways, too. They burn alive with the flamethrower, they explode like a bag of blood when hit with a shotgun shot, they turn into mush when you shoot them with your tank's main cannon. You can even turn fat, which turns your guns even more powerful, even going as far as changing how your projectiles look. Being fat has it's own set of idle animations too, for god's sakes.
The craziest bit has to be that when you see the credits roll, you are met with all sorts of nicknames that don't lead anywhere. No one even fucking knows who these guys that developed the games are, apart from a few key developers, and even they don't remember all of them or who they were.
It baffles me how a game series has managed to have fluid and overly complex animations, solid gameplay, a good dose of humor, extremely catchy music, complete John Does behind it all (or extremely hard to find out about), and it still manages to be fresh even to this day. This is the benchmark to which I hold all games.