I would have to say my all time favorite is Eureka 7, followed by Clannad. What's yours?
edit 1: holy crap over 1000 replies! I see we have tons of otakus here. Haha! Keep it up!
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Gurren Laggan Eureka 7 SAO season 1 Gundam Wing Outlaw Star Trigun Yu Yu Hakasho These animes were all 10/10