Honestly I have seen a frick ton of sad anime and I'm surprised to say this but the most emotional anime for me is Angel Beats. I guess it could be due to the shortness of the series that really makes it feel like that 'short but sweet' sorta vibe. Not really that but in that short span of episodes I really loved all the characters and the mix of drama/comedy that Key gets right ALL of the time. Like somebody said a few posts ago in here the whole
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really was bittersweet and just one of the most happy/sad moments in anime I've ever experienced.