The most controversial anime that I have seen is Kodomo no Jikan. This anime is about a third grader named Kokonoe Rin who falls in love with her teacher, Aoki. Kokonoe then teases the poor teacher to no end. Its quite a funny and sometimes deep story if you can stomach the fact that a child's in love with an adult. The anime itself is clean thankfully aside from a few innuendos.