Undertale music
While Megalovania is definitely one of my go-to songs from the OST, Bonetrousle actually does a better job of tying into the game proper by using pre-existing motifs, and conveys the emotion of the situation better.
Megalovania is awesome because it comes out of nowhere and hits like a MAC Truck, but it has no pre-existing chords from earlier in the game, so while it makes for an epic encounter, it isn't clear what emotion is being played on or amplified by the music.
Undertale music
Bonetrousle takes a goofball song and makes you go "Oh shit... I may have underestimated him." It subverts expectations. And for a song so early in the game, it's quite impactful.
Undertale music, spoilers
In terms of the Genocide route, I would argue that Battle Against A True Hero is a far more impactful song because it takes Undyne's theme and takes it out of minor chords into major, literally mirroring her switch from villain and rival to the last hope the underworld has against you, someone who literally just tried to kill a child. It gives me chills.
Undertale music, spoilers
@Sandrockcstm diegetically, battle against a true hero DEFINITELY is the stronger song. It reframes Undyne as the hero of the story, and I would say that you losing there is probably closest to narratively complete the genocide route can get
whereas the sans fight only expresses one emotion: no emotion. It's about exactly what sans says: You're here because there's content to see. Getting to the fight is boring and grindy. And if you're there, you clearly don't care about the shock value either
so he obliges, with content. With all the lack of actual emotional stakes that implies--a banger song and fight that might as well be a gaiden episode.