Although it was a less than perfect anime, Kanon 2006 has my personal favorite OST. The piano and synths really fit the mood of the anime, and sometimes I feel like I'm listening to the songs more than watching the story, because I like them so much. However, the Opening and Ending songs were a bit meh.
Higurashi no Naku Koro Ni is a really well written story, if you can stand the sub-par animation. Also keep in mind that it won't make any sense until the second season, but it all comes together very nicely in the end.
