For some reason I seem to find more ending songs I like than openers. I guess my favorite opener would be a toss-up between the opening theme for Magic Knight Rayearth and "Shell" from {b]Witch Hunter Robin[/b]. I also liked the openers for both seasons of Ghost in the Shell: Stand Alone Complex.
My all-time favorite ending song would be "Mizu no Madoromi" from Fantastic Children, with "Hime Murasaki" from Basilisk and "The Beautiful World" from Kino's Journey as runners-up.
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You look at it, I'm bitter.