I'm not sure what you would consider classic, but I think of classic as manga that has stood the test of time and remains popular years after it has ended. For me, these are
Banana Fish - Possibly the greatest manga I have ever read. I have read it more times that I can count.
Hana Yori Dango - Usually I hate manga like this one, but hana yori dango is so good it is impossible to hate, even for cynics like me.
Sanctuary - With the exception of how women are treated, which is kind of key to the style of the story anyway, I love this.
Angel Sanctuary - I'm slightly biased about this one as it was one of the first anime I ever saw and the 3 OVs weren't nearly satisfying enough, so I started reading the manga. In some ways, this was my first manga, so naturally I treasure it. However, I do believe it has a good balance of depth, action, emotion and complexity.
Fruits Basket - This is both funny and tragic and inspiring and entertaining, just a brilliant manga.
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