AC4 Black Flag

Final Fantasy VIII, I think. I can't recall a single thing in that game that I haven't done.

I made pretty good progress completing Breath of the Wild, but never got quite enough Koroks and Dragon parts. This thread inspired me to go back and try for that.

Otherwise, probably one of the SMT games that requires multiple playthroughs.

Probably FFXII. I did Yazmat, the entire bestiary, the zodiac spear in the random chest. I was only missing a few very rare drops IIRC.

Did everything in Persona 5 royal, also got 1000/1000 achievements.
Final Fantasy XII and XIII also did everything in those games

I am 40 and I have played gazillions of big games, I could not possibly say which was the biggest.

Demon's souls on ps3

I've never 100%ed a game where that would impress anybody. As a matter of principle, I do not engage in nonsense, tedious crap during an activity that is supposed to be fun. So, that it makes it impossible to 100% most AAA games these days.

I play all games to the extent that they are fun, and no more than that.

In fact, I'd go so far as to say that if you are 100%ing AAA games regularly, you need to re-examine your life choices and/or talk to your psychiatrist about your likely OCD. 

Breath of the Wild and Tears of the Kingdom. The only thing missing is fully upgrading every armor in both games.

That’s such a fun question! For me, the biggest game I’ve ever 100% completed was The Witcher 3. It took a lot of time, but it felt so rewarding in the end. It’s always cool to hear what games people have fully finished it really shows how much we love them!