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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Sword Art Online Attack On Titan Naruto Naruto Shippuden Yu-Gi-Oh Pokémon The Irregular at Magic High School The Devil Is A Part-Timer are some of my top favorites, SAO being #1