The episodes remain highly rated until the last season, although those of us who had read the books were complaining more before then - for me it was the combining of characters in a way that created plot armour, which ruined a big part of what made GoT great.
I think season 4 was the peak, and it looks like that’s a common opinion: https://www.ratingraph.com/tv-shows/game-of-thrones-ratings-26649/
The episodes remain highly rated until the last season, although those of us who had read the books were complaining more before then - for me it was the combining of characters in a way that created plot armour, which ruined a big part of what made GoT great.