Okay, so ignoring Route 10, my absolute favorite song from Black and White has to be Undella Town's theme during Winter, Autumn, and Spring.
https://www.youtube.com/watch?v=EuJd8D5Dh6A
I find the song incredibly soothing, relaxing, oddly romantic, and incredibly comforting. It was the first song I bought from the Soundtrack on iTunes, and I don't regret it one bit. The soft bells pair perfectly with the sounds of the waves crashing against the shore, and it's just beautiful.
I don't remember much from Black and White 2's soundtrack, but one that's oddly stuck with me has been Route 19's theme.
https://www.youtube.com/watch?v=1-TGFsfurG4
Yeah it's a first route theme, but listen to it! It's so upbeat and fun. Unlike any other first route theme, which normally gives off the feeling of going on your very first adventure (with the exception of Sinnoh's "welcome back" Route 201) this gives me the feeling of a veteran starting a new adventure, and it's absolutely wonderful.
...I hope that made sense.