Mysterious Dungeon definitely makes tops. That had some of the most joyful, filled-with-life music I've heard in a pokémon game. I loved the string music, particularly the city square theme, as well as that forest theme.
Also close to the top for me would be Hoenn, which, as with the original poster, is nostalgic, as Sapphire was my second pokémon game. Colosseum also deserves a mention, having a really catchy main battle theme (almost to an ear-wormish extent).