Diet drinks without caffeine are preferable if you already get caffeine from elsewhere IMO. Also certain artificial sweeteners are better than others. So you might want stevia over sucralose over aspartame.
I've heard some pretty good reviews about the organic fruit-extract Vitaberry that can be mixed with water and has minimal carbs. You can even grab it from NutraPlanet with your next order, only 4g of carbs per serving and they're naturally occurring (15cals/8oz). I've got some en route that I'm excited to try.