Coke (and other soft drinks, let's not single out one) are just bad for you, regardless what kind you drink. There are serious questions about aspartame in the diet drinks, but before heralding regular Coke is better, keep in mind it generally doesn't use sugar as the sweetener, but rather, high fructose corn syrup. This may depend on where you live -- in Europe, you are more likely to get sugar -- read the ingredient list carefully.
HFCS is evil -- while it seems like a natural product and no worse than sugar at first, fructose is processed by the body very differently from sucrose. It has to be entirely metabolized by the liver, is converted to fatty acids much faster than sucrose, and is known to contribute to developing of diabetes. It also increases the excretion rate of essential minerals.
So, if you drink soft drinks, you get to pick your poison -- literally.
Getting back on the subject of the thread, a few things I am addicted to include:
1) Tomato juice -- love the stuff, have to have some every day. It's been this way ever since I remember being conscious.
2) Information -- this one is starting to concern me. If I haven't checked the latest on my regular RSS feeds, forums, IRC channels and similar places for a few hours, I become anxious. I probably should take a two-week vacation on some uncivilized island that features no broadband, WiFi or cellular service.