The original Metal Gear Solid on PS1 is still surprisingly playable. It was an INSANELY "before its time" game back then. The graphics, the cutscenes, the music, the story and even the gameplay from those early says of 3D being a new innovation still holds up. So if you wanted to, you could just play it in an emulator.
But if you want to play it on a modern system, the entire series just got re-released (well, like half of the games so far).
I would recommend just following the numbered releases and just playing Metal Gear Solid games. The Metal Gear (no solid) are much older and don't hold up as well gameplay-wise, imo. I'll probably get yelled at or something for that, but I played the OG ass MG game on NES as a kid. Most NES games sucked then and still suck. I'm sorry!
So I would just play MGS1, 2, and of course 3. 4 is also good, but imo, that run of 1,2,3 are the golden trio or whatever. 5 is good too, but maybe just because I'm old af and have nostalgia, I definitely prefer the PS1/PS2 era ones far more.
Due to the non-chronological order and the mysterious nature of the games, a person could conceivably play them totally out of order and piece together the story. If you wanted to go chronologically then I think MGS3 goes back the furthest.
Just be ready to enter into a crazy universe. Once you get into MGS you never get out (đź‘€)
I literally can't even begin to explain the whole story, but suffice to say, a lot of events happen way back to like the 1960s/70s/80s and into at-the-time-present 1990s, 2000s. You get the opportunity throughout various games to play characters at different times in their lives. You get to see stuff or do stuff that is referenced later on in other games and be like "oh yeah, I did that thing!"
Just fire up MGS1 when you have a couple hours to set aside and if you aren't enjoying yourself within the first couple hours then maybe it's not for you. Which would be fine (I suppose). You could also probably find a decent explanation for the entire game series story online, but tbh, it's more fun to just play and fill in bits later if something was confusing. The game lore is ridiculously deep and not always fully explained in game or by Kojima outside the game (original writer, creator). So yeah, just try it, I don't think you'll be disappointed. The games are literally ranked in almost every top 10 or top 100 or whatever games of all time for a good reason. There's something in them for everyone and the total package is just fun