Honestly that’s really hard to tell and I feel like people will give different answers for different reasons. I personally think it can’t be credited to just one year because a lot of my favourite animes were released 2014 onwards 
I think recent times have just been good for anime in terms of technology to improve animation and of course experience. This isn’t to say anime before that time wasn’t good! There were some super good animes before that year.
@AnimeNerds come answer!