Halo

for me the decline started with the introduction of boss fights in Halo 2. CE blew my mind with zero 'supervillain' level boss fights most games that i played at the time resorted to. i really liked the concept that the three big antagonists were an advanced civilization of never ending zealots hellbent on ending the universe for their religion, then there's the hive mind of mindless swarming parasitic alien lifeform hellbent on eating/becoming the universe aka its own religion, and the ancient remnants droids that watch over it all like an endless production line metal monks guarding and maintaining said remnants like a religion...

anywho by the time of 5 the "villains" were so Marvel level of cheese/boring i was willing to press halo's big red button and just be done with it all.