This one is tough... Most of the popular anime are worth recommending, but if I could choose, I won't recommend Junji Ito's collection or other gory horror anime? It's only popular among a certain audience, but it might be disturbing for most people. Uzumaki by Junji Ito anime has just started... I haven't watched it, but I'm planning to. :) hahaha

Aside from horror... I wouldn't recommend Prison School. I personally enjoyed it, but others may find it offensive... and it's definitely not for kids. haha