I love Gen V's music way too much. The music itself is catchy and memorable, and Gen V's soundfont is possibly the best soundfont of any game I've heard. Favorites of mine include most of the battle themes, Virbank Complex/Castelia Sewers, Virbank City, Driftveil City, and Abyssal Ruins.
If you want me to gush about anything other than Gen V for once, ORAS's rendition of the Frontier Brains' battle theme and the Battle! Ultra Recon Squad are some of my favorites too.
I have fond memories of Mystery Dungeon, too. Lower Brine Cave, Temporal Tower, Fortune Ravine, and Barren Valley from the Explorers games are some of my favorite tracks, and even though I've never played GtI, Rescue Team DX's Oddity Cave medley is SO good. In fact, Rescue Team DX's soundtrack is amazing in general, because it manages to strike a good balance between the bit-crushed goodness of the original games and the sound quality worthy of a remake.