Yume Nikki's OST will probably go down as one of my (if not the) favorite OSTs. Even if they are just 4 or 8 bar loops, there is something so mesmerizing about them. Truly goes to show you dont need a lot to make something great. The track I linked is my favorite out of the bunch, love it to death.
As for anime OSTs, I have really been enjoying Hikaru no Go's one (as I am currently watching it). Welcome to NHK's one comes to mind as well, liked that one quite a bit.
