Pokémon always hits the nail on the head when it comes to music, so it'd be pretty hard to pick which generation had the best music. I think Gen 1 is still a strong contender for best music as a whole generations (Wish I could say the same for the games themselves though). My favorite songs I'd say
Alola Champion Theme
Diantha's Theme (2nd best champion theme behind the above mentioned)
Route 113 (Hoenn Original Version)
Lilycove Musuem (Original Version)
To give a few names.