For me it was Pokemon.
Started in order and got the games new as they came out since DPP. The core of the game to me was exploration, Pokemon, then battles. However over time Battling got more and more popular to the point where the game just became boring. This was Black and White for me.
I hoped it would get better with each release, but it was Sun and Moon where I just couldn't care any more. After that I tried a game here and there. But nothing ever went back to that core of Exploration, new Pokemon the battling.