Ima throw my vote in for gen 4. Sinnoh's soundtrack was great. There was almost no reason for me to ever go to canalave, but that music has stuck with me for eternity.
https://www.youtube.com/watch?v=e3dpyBZlXbcProbably my least favorite was X/Y/ORAS. Something about making the switch to the fully orchestrated soundtracks felt really off, soundtracks used to define locations and moments in pokemon games, now they just kinda fade into the background like they're supposed to.
