I honestly think that the music in the third generation is the best in the main series; only the latter half of Sun and Moon comes close, with much of Poni Island comprising some of the best in the franchise as well.
There's something adventurous, ambitious, and exciting about it - it really feels like your in this amazing far-away region, exploring the vast oceans around it and experiencing its highly varied landscape. It all just fits, really well. It's not quite super-retro like the generations before it, but it's not as modernized as everything from Gen 4 onward - it has its own vintage niche, and its own personality that's hard to blend with anything else. It's distinctive and highly thematic.
It's one of the pieces of the Hoenn equation that makes it probably my favorite region and that makes the third generation one of my favorites.