I liked almost all of the D/P/Pt music. The ones I liked the most are Mt. Coronet summit, Spear Pillar, Distortion World, Eterna City, Lake Verity/Valor/Acuity, Lake cavern, Lake trio battle, Dialga/Palkia battle, Giratina battle, Eterna Forest, Team Galactic boss Cyrus battle, Champion Cynthia battle, Canaclave City, Arceus battle, Route 207, Route 201, the snow route, Rival batte, Sandgem town, Twinleaf town, and Pokemon League.
I also like the Celestial towr music from B/W. This music sounds alot like the Legend of Zelda: Link to the Past light world dungeon. I playt that zelda game on virtual console.
I have a thing for Team Plasma's theme. It may be a bit annoying at start but it definitely grows on you. It did for me at least. And I also liked the Cipher Admin battle theme and Evice's battle music. Lastly, Mysterial's theme is just epic! Mysterial's theme is probably my favourite in-game music in a Pokemon game.
FR/LG- Gym Leader and Champion battle theme. And I chose FR/LG instead of RBY for a reason.
RSE- Almost everything.
HG/SS- Team Rocket theme. (Again, I chose HG/SS instead of GSC deliberately).
B/W- Cynthia's theme, Standard Legendary Battle Music (i.e. not for Reishram/Zekrom/ect.), Gym Leader theme, Lostlorn Forest, Rival Battle theme, Team Plasma Battle theme, N's battle theme (especially the last one) and, for the most glorious music every in history ever: the end credits music. It makes it even better seeing Reishram/Zekrom blast off into the distance afterwards.
I found pretty much all of the music in B/W to be very refreshing and I liked all of it, so I can't pick one from there.
Gen 1: The route 1 music is a classic, and will remain as one of my favorite tunes throughout the franchise.
Gen 2: The Cinnawood island music is so calming and nice, although I do like the Olivine tune quite a bit also.
Gen 3: The route before fortree city always stuck with me and made me feel excited
Gen 4: Once again, the route 1 music. Made me feel energized.