In the old forum, I posted a thread arguing that The King of Fighters '96 had the GOAT fighting game soundtrack on its 20th anniversary. I still think that's true today so lets put that aside when I say The King of Fighters '99 also has a GOAT fighting game soundtrack. I would argue '2000 is a close tie but I think first impressions matters a lot more than what is essentially a reiteration of a known successful formula.
The King of Fighters series has gone through a lot of makeovers over the years but nothing has been more drastic than the darker and edgier turn the series went with '99. Beyond it's visuals, presentation, and plot, nothing would probably represent this thematic shift more than its soundtrack. The cheery and more diverse tracks of the Orochi Saga games are out and in their place are the harder club-like beats and moodier tracks of the first entry of the NEST Saga. IMO none of the later KoF entries ever took a more drastic turn musically than '99 did.
For example: Here is the theme for the new protagonist K' and his team of genetic supersoldiers and Team Japan transplants:
Instead of the funky or upbeat themes of Kyo's Team Japan, KD-0079 hits you with some hard-hitting music. Music almost completely unknown in fighting games at that time. IMO this track ultimately represented the tone SNK set for the NEST saga (at least before they went bankrupt).
Team Fatal Fury's new theme 176th Street feels more in place for a jazz cafe than a fighting game:
Gone are Team Psycho Soldier's idol-musiclike/chinese-influence tracks and in comes the aptly named Psycho Sonic Trip:
The most drasticly changed in tone would arguably come from Kyo's '99 theme Tears. Symbolizing his departure of the role of the main hero of the series, here comes a track that invokes the iconic Esaka? from '96 but in way that seems nostalgic and longing for the good ol' days. Not a surprise if you know what he's went through after the end of the Orochi Saga. Arguably one of the best tracks in the series:
Here are some other tracks from the game that I quite like:
Sha-La-La
The Way to Rebirth
Ryu-Ko
And some official remixes of some '99 songs from later games that I want to give shout-outs to:
KD 0079+ (From 2002 UM)
KD-0079 -KOF XIV ver.-
W.W.III -KOF XIV ver.-