The worst anime series will always differ from one person to another but personally, I dislike Eiken. This is an anime series that will leave you distracted because all of the women characters just have very large breasts. How can you concentrate on the main plot of the story when you get distracted by the characters? It can be bearable if this is the only issue but the more that you watch it, the more that you will realize that it does not have any actual story or the main goal of the story does not seem to be too clear.
There are also a lot of suggestive things that can only be considered as inappropriate. I think this is the reason why I dislike it. I feel that I do not want younger people to watch it as it may give them the wrong idea about what is proper.
Given the fact that with all good things come equally bad things it is no surprise that while there are several amazing anime, there are also absolutely terrible anime as well. The first being Boku no pico which is a Japanese shotacon yoai hentai (this means that the anime has explicit content and is based around underage homosexuals).
The list after that varies from person to person as all preferences do. Others that can be found on this list are Eiken, Bakugon, and School Days. You also cannot forget to include Diabolik Lovers to this list.