The second game I ever 100%ed was Arceus and I still can’t stand the thought of picking it up again, years later. The first game was Horizon Zero Dawn, which is still fun.
If I remember correctly to 100% Mario Odyssey you need 999 moons. The game gives you 850 or so normal/good/fun moons, but the last 150 or so you have to buy with coins. For me this meant grinding a flower challenge in Bowsers Castle ~20 times in a row.
I did it, but that part wasn't fun.
The rest of the game is perfect, but that part would keep me from 100 percenting the game again.