MGS1 is my favorite of the series and is one of my favorite stories in gaming. It's an incredibly mature story for all the wackiness thats still in it. The music is perfection. The performances are incredible. The characters are great. I love everything about this game. I think, somehow, the low poly graphics and textures adds a lot to the realism and grittiness of the game, despite being fairly arcady in gameplay. At this point in the story, almost everything that happens in it is believable, other than the supernatural parts (which are, in this game, actual supernatural powers, which adds a whole new layer of mystery!) I HATE how over explained everything became in MGS4 and MGS5, (and to some extent, mgs3 even.) I think MGS4 kind of ruined the series for me, even though mgs4 has my favorite gameplay. MGS5 just felt like a Ubisoft collectathon game to me, and vocal parasites can lick my ass with how dumb it is. It has good moments, but it feels rather aimless compared to MGS1-3.
1 has the most serious tone, but still has some goofiness and arcadiness that really meshes well with the story and visuals somehow. 2 and 3 stayed strong with the story, both being more goofy, but also having deeper meanings and plots. I like 1-3 a lot, and even 4 is a fun romp, but 5 just could not hold me. Ground Zeros was great! But Phantom Pain is just a slog for me.