Sinnoh will likely always be my favourite region. I forgive it for its bad points, and fully acknowledge it can be a chore to get through due to the slow pacing, but I feel in some way that represents Sinnoh's atmosphere( not intentionally of course).
It's very relaxed in Sinnoh. The music has a nice influence of jazz that really hits that nail home, and coupled with the night soundtracks, you really get a feel how easy going this region is, despite the epic lore behind it. The best example is the Pokemon League music. Its some of the most laid back in the series, but your about to take on the blooming elite four!
Its the reason I love Sinnoh, it has atmosphere in buckets and buckets. The route design much more complicated than a normal pokemon title, and makes exploring even more fun, especially with all the weather effects. You can tell its a chilly, rugged, mountainous place, and makes you feel comfy when you get to one of the awesome towns. The graphics add to this with it's charming pixel art and colourful palette. I love how each town felt unique, but yet part of the same region, and the town music is my favourite in the whole series. It may not have the best gameplay or dex, but it's easily my favourite region.
Eternal Diamond and Cosmic Pearl can't come soon enough.
Edit: Just to sell my point, here are some choice tracks -
Route 216,
Jubilife City,
Pokemon Center,
Pokemon League