For me, that title belongs to The Legend of Zelda: Tears of the Kingdom. All 1,000 Koroks, the Hyrule Compendium, every single (+every chest in each shrine), all temples, explored 100% of the map (according to the in-game tracker), every single chest in all three layers of Hyrule was opened, every single-spawn items was collected (and displayed in my Hudson home), all the side quests, …and everything else. Took around 395hr total.

Final Fantasy X International. Not quite a 100% though, as I didn't play the Expert Sphere Grid mode or whatever it was called.

Assassins creed black flag - loved all the deep diving, sailing, and discovering on islands

Watch Dogs - I don’t remember what I liked so much about it but it did have me coming back

The collector in me enjoyed knowing I acquired all the everything

Halo 3, got ever achievement

I Usually do not 100% games, but I did for Hyrule Warriors Age of Calamity.

It took me 100 hours to get to 99%, just to discover that I needed to find an additional 11 Koroks to get to 100%. It took me an additional 30 hours to find them, because I was to stubborn to use a guide.

Probably GTA IV. But because of stupid bugs the platinum didn't pop. =P

It did though with FF XIII. Which was also quite time-consuming.

Still haven't 100% completed The Binding of Isaac: Repentance, even after 1000+ hours. But I can't be bothered to even try to get everything with Jacob & Esau, lol.

I guess Breath of the Wild, though not sure what constitutes 100% here. All Korok Seeds, all shrines, all side quests, all memories, 100% of the map was what I got.

If not that, then probably Wind Waker. I once got everything on the GameCube version, including buying a cable to connect to the GBA just to use the Tingle Tuner and unlock Knuckle.

Final Fantasy Tactics A2: Grimoire of the Rift. I did every possible mission and event. It took me about 200 hours or so.

The original Hyrule Warriors.

Legend mode on all difficulties, all five adventure maps, challenge mode with gold for all 19 characters, challenge mode with Ganon and the chicken, and all 21 of these characters at max level 250. All 19 regular characters had ultimate weapons for all the weapon types they could use, and all achievements were unlocked. I think that took around 320 hours in total.

Later on, Hyrule Warriors Legends was released for the 3DS with additional content that was partly added to the Wii U version as well, so my 100% file got knocked down to incomplete.

I did purchase Hyrule Warriors: Definitive Edition for Switch (I skipped Legends on the 3DS), but so far I am only around 50% through its adventure mode and therefore still have a long way to go. I estimate that I've played under 100 hours and the whole thing will take around 500 hours to get everyone ultimate weapons and level 250 on top of finishing all modes.

Honestly, I don’t think I’ve ever hit “100%” on a game that tracked it.
But I haven’t had much luck on that front either. I don’t think I’ve ever come across a game that hasn’t required some collection of bullshit tedious tasks to get it.

However, I have done some shit in FF8 that most players would never do… like crafting scrolls and customizing GFs to make my characters stupidly powerful 😁