Europa Universalis IV, 244 hours. I don't think I understood a single thing of what was happening. And with every free update, some of the rules changed, sometimes in unpredictable ways.
I was told "don't worry, even the big-brains who plan out their entire campaign with spreadsheets don't really understand the mechanics". But that's no consolation, I felt stressed and confused for every single one of those hours.
May I just say that it's so nice and refreshing to see lots of people mention these supposedly amazing 10/10 games in this thread and hate them, I largely feel the same about all those titles. I used to think my taste was just broken.