Gen 5 was great at establishing characters using the music.
Alder's Encounter
https://www.youtube.com/watch?v=_KE1iXYzRmwDude's a wise mountain man, and this music shows off his adventurous and light-hearted nature; shame his Champion music was more generic in comparison.
Champion Iris
https://www.youtube.com/watch?v=BrKeGHwOBvsAn energetic 10 year old girl is the strongest trainer in the region, and the start to this song is so perfect for that. Has a great buildup and is fun and upbeat.
N's encounter theme
BW:
https://www.youtube.com/watch?v=73yPZOiLDFoBW2:
https://www.youtube.com/watch?v=4l7jZTYYMJwI like these especially because of the progression from BW1 to BW2. BW1, his music is creepy with a weird rhythm to it, because he was creepy and off-putting.
In BW2, he's a completely different person but still sort of an outcast, so I love how it starts with the original theme but then becomes so much more lighthearted, since he's a happier person after BW.
VS Ghetsis
BW:
https://www.youtube.com/watch?v=nX2hOoaLb4YBW2:
https://www.youtube.com/watch?v=sjueMDjuqm4In BW1, Ghetsis is an evil mastermind and one of the more successful, low-key, and manipulative villains in the series. He's a real threat, and the music and choir that back the battle fit his intensity.
Then in BW2, he's gone fucking insane, and his music and the choir that opens the song shows that strongly. Sort of the opposite shift that N's music went through.
And I don't think anyone put this so I'm throwing it in here:
https://www.youtube.com/watch?v=x3ZUBRfdKB8 
