My first time playing Final Fantasy VI (III in the US), I started playing it at 5 in the afternoon, just kept playing it, got to a stopping point, and realized that it was 7 in the morning. I was 16.

Vaguely, I remember spending the next day hearing the music roaring from the heavens interspersed with my mother yelling at me for sitting up all night playing video games. Thankfully, it was a school holiday and we lived out in the country, so I didn't need to account to anybody other than my parents for my admittedly bizarre behavior the next day until I finally slept it off, and my parents were rather resigned to the situation by that point.

12 hours give or take a few times (with 15 minute pauses for food).

I got 4 contenders.

The first one was Panzer General 2, where I played from Guadalajara through Poland, Finland, Dunkirk, Windsor (Operation Sealion), Greece and Kishinev. Took me about 18 hours to do so.

The second one is Fantasy General, where I played through continent 3 all in one sitting. My units were so OP after the middle of that session as I had fully researched light and heavy infantry playing as Lord Marcas (who gets those units bonuses) that even with level scaling of the enemy troops they were no match to my elite troops.

The third one is Oxygen not Included, having played from in-game day 1 through day 118 in one go. Still probably shorter than the two above though.

Lastly, one special case: Super Smash Bros Brawl on the Wii. It had problems to read with my Wii, so when after countless times trying it finally worked, I simply left it running as long as I could, playing through the entire story mode and some other modes in one sitting to be sure I could get the most out of it working for once.

Now which one of those 4 is the longest sitting, I don't know, but each of these are at a minimum 14h straight gaming.

The only time I can recall is one time playing Heroes of Might and Magic III with my brother. Started around 19.30 and ended around 10.30 the next day. Also the only time ever I have been awake for over 40 hours straight.

Between 76 and 96 hours in Minecraft, there was breaks but it was just doing shit in my apartment like the dishwasher and clothes but I was mostly glued to Minecraft. I got into a walled off server that tested you for building something to get in to make sure you were up to their standard with their own themed blocks. I took thst mission to build something very seriously and my the time I had been accepted in three full nights had past and I was very close to hallucinating. Not a chance I'd be able to make it through the first night anymore, I don't know what happens between being 21 and 31 but you loose a lot of resilience to some very basic things. I'm lucky if I have the energy to boot up a game late in the evening now.

Multiple all nighters when I was a teenager playing so many different games.

But the main ones were wow in my teens and LoL in my 20s.

I would get out of school at 3, start playing and not sleep until the next day after 12. sleep until dinner and do another all nighter. Then feel like death during the week.

I think the longest I stayed awake was only 30 hours, and I was probably playing wow

I pull 12-16 hour stints pretty regularly, with nothing but bathroom and food breaks. I don't think I have ever gone on for a full 24 hours or something like that, though.

With bathroom breaks and 5-10 minutes to eat, 15 hours. It was an endless shareware strategy game where you couldn't save, so you just had to keep going if you wanted to make most of what you had build.
My buddy and I were kind of competing on finishing management strategy game Detroit, so at one point I did 14 hours on that.
And my third longest is Dune 2 with something like 12 hours.
I'm sure one or other Civilization gets to same category, but that's with more breaks and doing stuff on the side.

On actual one sitting with no break whatsoever, I think I have at some point played Pokemon for 7-8 hours.

On action games with no breaks over 10 seconds, I have played Master System Hang-On and PC version of old Star Wars Arcade game for over 3 hours straight on.

Probably never more than 10-12 hours with breaks during the day, my body very much screams 'enough' at one point.

My brother pulled a couple of all-nighters for some MMO quests when he was younger, though, which was crazy to me.

If no breaks are a pre requisite, no toilet breaks or anything then 8 hours on the MGS5 bosses when the helicopter crashes, the . I know cause I started profusely sweating cause I was so immersed in the game that I forgot to piss. They weren't even difficult just annoying to fight and you had to chase them down over a small map. Really frustrating. After that probably about 6 hours on the true Owl boss in Hirata estate in Sekiro, now that was difficult, I had already spent hours on him in the days before but I said I'm not budging until I beat him, took about 6 hours.