 If you liked Great Teacher Onizuka |  ...then you might like Sayonara Zetsubou Sensei |
Both shows feature a teacher as the main protagonist surrounded by problematic and troubled students. The gist and basis of both shows are p. much the same. Although both are classified as comedy and parody shows, the style of humour in both shows are very different. The protagonists too are quite the opposite personality-wise. However, if you're a fan of comedy anime and liked any one of the 2 aforementioned shows, you will probably like the other.