I'M GOING TO BE UPSET AT YOU FOR A FEW MINUTES, LJ, BECAUSE THINKING ABOUT SNAKE HAS GIVEN ME FEELINGS I MUST EXPRESS.
Over the course of the games, he grows so much. MG1, he's not much of a talker, or a character. He's just an obedient soldier who follows orders and is betrayed by his commanding officer, who is also his father. MG2, Snake becomes more of a person, though still very given to following orders. He's a little messed up at the start from the previous game, and is even beginning to show warrior poet tendencies. He trusts and is betrayed by several people, and has to kill his father and his best friend. By the end, he'd be happy to walk away from it all.
In MGS1, Snake's PTSD becomes more clear. He spent years isolated in a cabin between this game and the last, and it took considerable goading to bring him back for one last mission. It's in this game that he becomes a full person compared to his 2D self, a consequence of his new 3D medium (perhaps). He feels conflict over killing- his guilt versus his perverse enjoyment of the act. He becomes emotionally attached to Meryl and feels the pain of losing her, and he gains a friend in Otacon. He confronts specters of his past in Gray Fox, Liquid Snake and Metal Gear, and wrestles the issue of his destiny as a soldier being in his very genes. But by the end, regardless of whether it's the Meryl or Otacon ending, Snake breaks free from the shackles of following orders, of being commanded by his genes or his past as a soldier or even FOXDIE. It's pretty clear that as the end of what was supposed to be a trilogy, this could have been the end of Snake's arc, with a bright future after the death of his clonebrother and the destruction of Metal Gear Rex.
But MGS2 brings Snake back, and it's here that he's at his best. Snake is fighting for a cause of his choice, alongside someone he trusts and loves and who shares his passion for combating the spread of Metal Gear technology. Though he still follows orders, they're from a friend watching his back, rather than from a commanding officer, and he's only a secondary protagonist with a minor segment of gameplay and being controlled by the player. Snake jokes and makes petty complaints and has wonderful banter with Otacon. Then he nearly dies and is framed as a terrorist. When he comes back two years later, he's more furtive and serious, and more of a philosopher. The tanker incident was a reminder of mortality, and that along with the thematic issues brought up by the Patriots S3 plan brought him to focus greatly on the idea of passing along important things. He spends a lot of his time coaching Raiden into thinking about this as well, shaping him into an independent being who can break free from his own destiny of being just a copy of Snake. And in the end, Snake's speech makes it clear that he has made the decision to pass on important memes*, as well as that he expects Raiden to do the same. As a great poet** once said, he basically said that he and Otacon were ready to adopt.
Which they did, in a way. Between MGS2 and MGS4, Sunny comes into the lives of Otacon and Snake by way of being rescued from the internet by Raiden. Snake's accelerated aging hits him like a freight train, and the rise of PMCs and cheap Metal Gear technology sends the world into an Ocelot-fueled dystopia out of Snake's worst nightmares. He is outdated, a relic of a previous age of warfare (and gaming), and in the face of such odds, Snake loses that self-motivated hope that so drove him in MGS2. In MGS4, Snake has all but given up, and truly wants to on some level. And yet he is dragged back into action for one last mission, because he is a weapon best suited to bringing an end to the other relics of his time.
In the face of what Snake was in the previous games, Old Snake is depressing as all hell. He rejects the affection of Sunny, not knowing how to handle getting attached to a person who he will die long before. He takes up smoking again after an apparent extended period of having quit, impervious to Otacon's nagging in the face of looming death. And even Otacon seems to give up, pursuing a misguided relationship with Naomi in a shocking regression of his development re: love, just in order to preemptively fill the hole in his heart that Snake's inevitable death will leave.
Raiden, on the other hand, goes from "cagey obedient dumbass" to "awkward but open about his past and free from the player" to "completely absurd cyborg ninja who is badass by virtue of having broken free from the player" to "whatever the fuck is going on in Revengeance/Rising". I don't know.
*In the sense of "basic unit of cultural information exchange", not in the "stupid internet copypasta" sense, dingus.
**
fireholly***
***Dear Fly, this post is dedicated to you for getting me invested in MGS meta in the first place, and also because I know you love Snake even more than I do.