I'm struggling because there's so many I like ;-;
I've always been a fan of gen 5's music, especially N's theme, Nimbasa City, The first team plasma theme, Village Bridge, and Undella Town. Jubilife city is also good. I also really like Lusamine's themes in Sun and Moon, also Po Town! Cynthia's theme, Couramine City, Emotion, Olavine City, and the Kahuna theme are good too.
Sorry I ended up rambling there's just so many good ones to choose from.
Route 113 from the original R/S/E is my all time favorite. I wasn't really feeling the way they updated it for ORAS. The abandoned ship theme from gen 3 is good too.
I like the Cinnabar Island theme from HGSS - it's sweet and sad. Kind of along those same lines is the song that plays right before you enter the Pokemon League at night in DPPt. I love that one.
I like Undella Town in Black and White... the fall/winter/spring version, where you can hear the waves. Village Bridge was neat. It was cool how you could change some of the instruments that were playing in certain locations in B/W, and how some music sounded different depending on whether you were walking or standing still.
I really like Laverre City and Malie City (day and night), but I have a hard time remembering most of the music from the last two gens. Besides the Poke Ride tune, which is not one of my favorites
