@Symphony of the Night - I totally agree about Lilium. I haven't seen the series yet (I'm planning it for months) but the song is just plain beautiful.

I'm also quite fond of music from Panty and Stocking with Garterbelt even though that's not my usual type of music, it fits this series very well. I also believe that a thread about best anime music can't go without a mention of the opening theme from Serial Experiments Lain "Duvet". That is a masterpiece.