I've recently been completely obsessed with Undertale music, but thats been mentioned many times before already.
I'm not a composer or musician of any kind, but I absolutely love it when OSTs re-use songs in various ways and keep the score whilst also completely transforming the song. Undertale does this so much, the 100 track soundtrack based around a 15 or so songs.
Pokemon has some really good music in my opinion and was the last time I saw this executed really well.
Pokemon Alpha Sapphire and Omega Ruby's remixes were fantastic, but my favourite theme itself comes from variants of entirely new track for the rival Wally.
The first shows off his personality pf being shy and timid rather well, and he has somewhat of a reputation among fandom of being one of the weakest rivals in the series.
In the remakes he eventually becomes one of the strongest opponents in the history of the series, being the trainer with the third highest levels amongst the entire series and arguably having the best team used by any trainer ever. This journey for him truly begins in the rematch in victory road, where he vows to become the strongest he can be and to defeat you, the battle being complimented by a metal remix of his character theme.