Sinnoh-based games I think have best soundtracks out of all.
I agree. Love that Sinnoh music. It's my favorite soundtrack too :)
My favorite themes include Dark Cave's (The HG/SS remake) and the Route 216/217 theme. Both of those are
awesome.
I also really like Cyrus's theme.
That bass. And my favorite champion theme is... Iris's! It's really unappreciated. I believe it fits the mood very well: a battle that is both friendly and difficult. Basically the song is saying "No hard feelings but, Iris is going the mop the floor with you today!" Cynthia's is very nice too. I like the variety of instruments and sinister organ feel.
I also like Lycr- Lycr- What's his name!? Whoever the Team Flare boss is (HE'S SO AWESOOOOME) his theme is rocking.
...oh alright, N's theme in Black 2/White 2 (NOT Black and White! His theme was so obnoxious in the original games!) was amazing. I also like the theme that plays in X and Y when your friends show up. And while battling Cheren or Bianca. And when battling your rival in Platinum. And the Frontier Brain theme in Pokémon Pearl/Diamond/Platinum. And the Elite Four theme in Black/White, Black 2/White 2. And Professor Sycamore's theme. And the piano remix of Professor Rowan's theme. And the gym leader theme in Pokémon Red/Blue/Yellow. And every single theme that plays when you run into a legendary Pokémon.
All of them are great.
Basically, I like every theme, some better than others, but overall they're all at least good. Except for N's original theme from Black/White and Arceus's theme. I also really dislike White/Black Kyurem's theme, Team Flare's theme and Ghetsis's Black 2/White 2 theme.
There are probably some more I don't enjoy, but I can't remember them...