FFIX's Heat was very annoying, since it was impossible to cure (although a Remedy/Elixir may have worked, I'm not sure) and tended to be inflicted after you'd chosen the character's move for that turn. Unavoidable KO's are not fun.

Stop's also right up there, as there's no curative item. Again, a Remedy or Elixir might do the trick, but I can't remember.