Specifically talking about Kingdom Hearts: Regarding the past, I have to say my favorites are Dearly Beloved in Birth by Sleep and Fate of the Unknown. Hearing Dearly beloved in blank points had an effect on me that has lasted for 7 years and will never go away. Fate of the Unknown had an effect on me as well because it was during the time when I was waiting for KH3 in 2007 and I just kept watching that godly KH2 FM secret ending. It was such a shift from what KH had previously shown in the series. That secret ending had an impact on me.
Now talking about this year, Yoko slayed KH 0.2 with Wave of Darkness and MoM's theme in Back Cover. That piano adlib during MoM's theme was so crazy. I loved it.
Now about another series: Everything Versus XIII.