All fizzy soda's (coke, pepsi, fanta, tango, 7up, sprite etc.), wether diet not, are not good for you.
They contain sugars, sweeteners, artifical flavourings, asperentine, citric acids... most of which are "fake" ingredients, manufactured for the drink.
Fresh juice, and sugar-free squashes are the way to go.
I also remember reading, a while ago now I might add, that the "diet" drinks i.e. Diet Coke in some ways are worse than the regular, as when you drink the "Diet" version, your body still thinks it's about to get sugar from the sweet drink; when it doesn't it then produces cravings for other sugary food to get that sugar that it's expecting.
I know for me, a few sips of Pepsi Max and I want every sugar-filled thing I can get my hands on, hence why I haven't drunk it in over a year now; before I'd even heard of LL's expression "trugger food", I had identified it as my main one.