which game has the best OST?

I agree with black and white

I can't choose! So here's a list of a bunch of OSTs I enjoy in no order:

XY: Literally all the city songs are gorgeous (especially Lumiose), like the cities themselves. Also the battle music is great, have you heard Diantha's theme? Also the route music. There are barely any boring or bad songs in Kalos.

BW: N's castle, both versions, is amazing, also Farewell, also Castelia city, also Driftveil, also many more other songs. The music is always enhanced by the beautiful pixel art everywhere.

SM: Gladion's songs (which are a remix of a PMD song btw), Lusamine's songs, Lillie's songs, Hau's songs, the town songs, the route music, literally everything is awesome.

The entirety of PMD. Dialga's Fight to the Finish is just one of many examples. If I actually list more I'd be writing for 15 hours.

I listen to video game music while working so I've heard many themes a lot

My favourite themes tend to be either Animal Crossing miscellaneous epic themes from star fox, punch out and Zelda, and then Pokemon S/V battle music (the best in the series)

@Hey_PIKMIN If we open it up to non-Pokémon soundtracks the list could go on for a while.

Some big hitters that rise to the top for me are the Bungie Halo games, the Zelda and Persona series. I constantly listen to tracks from those games throughout my week.

If you like more classical or orchestral music Jeremy Soule's work on the Elder Scrolls series is phenomenal. Recently I also liked the Baldur's Gate 3 soundtrack. I particularly like Down by the River and I Want to Live.

I have to mention the Sonic series. Like Pokémon, the music remains one of the few things fans agree has remained high quality throughout the series. Open Your Heart, Live and Learn, Escape from the City, What I'm Made Of and pretty much the entire Shadow the Hedgehog vocal soundtrack always get a look in whenever I go to karaoke.

Mentioning Sega games, anything by Hideki Naganuma is great. Particularly the stuff he made for the Jet Set Radio series.

Turning to indie games, Touhou is an absolute giant in this realm. Not only are the original game tracks great, but the doujin music scene encompasses every genre with remixes and covers by artists the world over. I have non-Touhou fan or even non-gamer friends who have come across some Touhou music independently and caught them listening to it, only for me to point out the original source.

You may have already heard a remix of a Touhou track in a game or beyond without realising it. Some indie games straight up jack Touhou tracks.

Just some of my favourite tracks and their remixes include U.N. Owen was Her, Septette for a Dead Princess, Lullaby for Deserted Hell and Beloved Tomboyish Daughter (English translations may vary for all these tracks).

That is just right now off the top of my head. There is plenty of games and tracks I have forgotten or neglected to mention, but I don't want this post to be too long.

Pokémon Black and White, although Pokémon Black 2, White 2, Ruby, Sapphire, and Emerald come close.