It all depends, how do you feel about artificial sweeteners? If you're fine with them, any Crystal Lite type of shit is fine.
If you want to avoid artificial sweeteners, but also aren't interested in calories or a high glycemic index, go for herbal tea (so no caffeine) with erythritol. It's a natural sweetener, has a low glycemic index, and has essentially no calories (they say it has a few calories, I'm not convinced of that given how it is metabolized).
If you don't like erythritol, same recommendation, herbal tea, but sub in Monk Fruit (also known as Luo Han Guo). It is another natural sweetener, has an ultra low glycemic index, and has no calories. I only recommend it second to erythritol because it is typically more expensive.