I agree with you about the ambient quality- almost makes me wonder if they were listening to Mogwai or Mouse on the Keys or even Harold Budd when they composed it. It's quite wonderful!I haven't listened to all the tunes in great detail, but I think my favorite might be 8 pm or 4 am. I really like those tunes.
Oh, and New Leaf has my favorite soundtrack by far. It's the most ambient. It makes great background music for just doing random tasks. The other games had good music too, but it all felt like "game" music. New Leaf's soundtrack doesn't have that effect for me, and so I love it all the more.