diet soda
I drink Coke Zero (Vanilla and Cherry, mostly), Diet Dr. Pepper, Diet Coke and Diet Mt. Dew daily. The only weight gain I've experienced was during my pregnancies and easily lost that weight after having them while continuing to drink my diet soda.
If you want it, drink it. Zero calories are zero calories - unless you're mixing it with someone that doesn't have zero calories. Will the artificial sweeteners make you eat things that are bad for you? I don't know. I'm not a doctor or a scientist - but one thing that will keep you from gaining weight whether you're drinking diet soda or not is keeping track of what you're eating and making great food choices (FOR FOODS THAT DON'T HAVE ZERO CALORIES, HEHE!)
Waiting for my 24 can case of Diet Dr. Pepper to get cold. I love grocery shopping.
I will say, though, that immediately post-op from my RNY the carbonation bothered me. I just poured it over a lot of ice to cut the carbonation down and I was golden.