Fizzy drinks are to be avoided altogether, but if I had to choose, I would go for sparkling water with a slice of lemon.
All sorts of sodas contain, among other things, sugar which is not good for you, and diet products usually contain aspartame that is even worse.
I LOVE sweet drinks, and finding it a bit hard to drink even tea without sugar at the mo. Have bought organic honey, but not sure how much of this I should have?
Hope I helped a bit.