You have two, maybe three, choices. Sprite Zero, Diet 7-Up, and Sierra Mist Free. Which you choose will be a matter of preference, but I always reach for Sierra Mist Free. To me, Sprite Zero has too much carbonation, and doesn't have quite as sweet of a profile as I like. Same thing with Diet 7-Up.

Sierra Mist Free uses the aspartame/Ace-K sweetener blend, and it works very well. This sits right up at the top with Coke Zero and Diet Dr. Pepper when it comes to tasting like 'the real thing,' and not a diet beverage. I drink both the sugar sweetened and diet version (you wont find Free in many restaurants), and I really can't tell the difference in flavor. Texture maybe; the no calorie version doesn't have the corn syrup stickiness.

You have some flavor varieties as well; there is Cranberry Splash (if you can find it), and a new orange variety is rolling out shortly. I've also seen a 'Free' version of Sierra Mist 'Squeeze.' Pepsi has never been a company that was afraid to experiment, so I'd look for more branches to shoot off the tree in the future.

These can be found almost everywhere Pepsi products are sold, in every popular size. If you're just getting into low calorie beverages, Sierra Mist Free wouldn't be a bad place to start, especially if you're a fan of this genre of sodas.

–Jeremy Hobbs