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.
Sekiro: Shadows Die Twice and Elden Ring and most other soulslikes.
Other games are unbearable to try 100%, for example: Cities Skylines where you have to wait for a generated special natural disaster that may or may not appear after X hours in your current savegame.