Diet coke has citric acid in it apparently so no good if you're in Ketosis x
The reason that Coke Zero is 'ok' (although not sanctioned by CD) is because it is zero calories unlike some other diet drinks and also it does not contain citric acid which can throw some people out of ketosis. There's a sticky about it on the main CD page.
Some people don't have any problem with citric acid whereas others will be coke zero is definitely the safer option.
BTW, Coke Zero and Dr Pepper Zero do NOT have citric acid but Sprite Zero and Fanta Zero DO contain citric acid