One of the worst I can think of is
Gypsy, the term is often used as an insult towards the Romani people. I also think
Ginger is kind of difficult.
Ninel (Lenin spelled backwards) is another one that has been in use. Lenina is equally bad.
Fanny is another one that doesn't work anymore at all. And I would hate to be called
Adolf or any variant of it such as
Adolfo or Adolfina :(