My personal favorite is Elfen Lied. It has great art, stunning music that you'll be hearing all day, and it gives me goosebumps after finishing the series (Which made me feel like someone cut out part of my soul)
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I'm very suprised no one posted these. [url=http://www.youtube.com/watch?v=zfLZk5KbVPA&src_vid=--ce1frD9rU&feature=iv&annotation_id=annotation_106490]Number two[/url] And done.