This all depends on your current diet. Sugar typically is bad fruit sugars are a little bit better however... your body will process and burn sugars and bad carbs first and then turn the stuff you consume that is more healthy for you into cholesterols. The body is a very complicated machine achieving balance can be difficult but well worth the work you put into it. Diet pop and regular pop of course is bad for you period the carbonation as well as the carbohydrates in it do not bode well for health mode. Not to mention have you ever seen what putting diet or regular pop on a battery connector will do? It's corrosive to the filth and rust on it...if it does that to an inanimate object you can only imagine what it would do to your stomach.
In Moderation everything can be "ok" you just need to keep track of everything you put into your body and eat more of the things you need and are healthy and then do very very small portions of the things you love that you cannot do without but are unhealthy for you. Once a month I go to chinese buffet and eat 1 of my crab rangoon and 1 piece of my fave tempura chicken nugget. But the same visit I eat lots of steamed veggies with a nice piece of salmon since salmon is high in monounsaturated fat that in your system works as a rotorooter for your vessels as it carts the bad cholesterols off to the liver to be turned into bile salts and then excreted.