Diet soda is either on the same level of harm as seltzer, or is worse. But I believe both to be healthier than any sugared soda.

Different diet sodas use different artificial sweeteners, so diet soda cannot be treated as a single group. Diet Coke uses only aspartame as sweetener; Coke Zero combines aspartame and acesulfame K to create a more realistic taste.

I readily believe that any diet soda sweetened purely with aspartame is harmless. The chemical structure of aspartame is such that stomach acids should break it down into aspartic acid, phenylalanine, and methanol, the first two being ordinary amino acids. Methanol sounds threatening, but very little aspartame is needed to sweeten a beverage. I thus have no problem drinking diet coke.

What's more unclear is whether the sugar-like taste of artificial sweeteners affect the body's blood sugar regulation.