Please bear with my english.

Sayonara Zetsubou Sensei is Not a Masterpiece. It's Not even great actually. Basically this manga is not for everyone. It's main genre is comedy and yet it's hard to understand that comedy. This manga's comedy is mostly about present japan i.e you have to know more about the japanese culture and about the famous people living in it. And people like me who knows almost nothing about japan had hard time understanding the comedy in the chapters. There are some chapters where comedy are only focused on the japanese cultures that I even skipped chapters. Yes, I even skipped the chapters. Ofcourse many of them were really funny because i love dark humour but still there are dull moments in the manga. You can get bored really fast because of it. Then why? WHY am I giving this manga a rating as high as 9? It's simple... twist.

Almost every popular or critically acclaimed animes/mangas has that one huge moment or that one huge twist that changes the show completely. Onepiece, the highest selling manga has that huge moment. hunter x hunter 2011, with its slow and weak starting had that huge moment in the later episodes which made it one of the best animes of all time. Sayonara Zetsubou Sensei has that huge twist that will instantly blow your mind and want to reread the whole manga again. Yes, all 301 chapters of the manga again. Its insane, it's weird, it's mind-fucking-ly brilliant, it is unexpected. It changes the view of the manga completely. Also the moral that this manga, "don't commit suicide" is successfully conveyed. And because of this unexpected event I just couldn't help but praise it.

So Story is a 6 because it's quite simple tbh.
Art is 10 because I don't know how to draw so idc much.
Characters are 8 because some of them you will forget.
Enjoyment is 7 because comedy was good but again not that great.

Overall- 9 because it will make you reread again.(but if it didn't make you reread again..........)