I second the NOW brand liquid stevia! No added weird stuff, they’re very transparent about the process they use to extract it (go to their website for all that info), it’s super concentrated so you don’t need much, and it’s the best deal for your money hands-down. Powdered stevia, unless it’s 100% pure stevia (rare to find, as stevia is so very concentrated), tends to be less ideal, as the added fillers tend to be problematic from a health perspective with no real upside (they don’t make it taste better or anything) other than the convenience of having pre-portioned packets on hand. I use the NOW regular organic liquid stevia for tons of things and the toffee flavor in my morning coffee. Stevia can be a bit of an acquired taste, so don’t reject it out of hand if you don’t like it at first. Sometimes combining it with erythritol and/or monkfruit in the beginning can help you get used to the taste until it’s pleasant. Adding too much stevia will make your coffee bitter, so beware of that as well.
https://www.amazon.com/gp/product/B005P0VL5S/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B00HA6Q6AK/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
I use the lakanto monkfruit/erythritol blend for baking and stuff like that. You can basically use it as a 1:1 replacement for sugar in recipes, which makes it super easy to use for such purposes.
https://www.amazon.com/gp/product/B00EUF5N1C/ref=oh_aui_search_detailpage?ie=UTF8&th=1
Pure monkfruit is better if you’re looking to avoid sugar alcohols, but, like stevia, it’s pretty concentrated, and a little goes a long way.
https://www.amazon.com/Lakanto-Monkfruit-Extract-Sweetener-Mogrosides/dp/B076DGNW88/ref=sr_1_3_s_it?s=grocery&ie=UTF8&qid=1535636895&sr=1-3&keywords=pure%2Bmonkfruit&th=1
Bulletproof coffee is pretty popular on keto, if that’s what you mean, and it can be satiating enough to keep you full all morning without any other type of “breakfast” without raising your insulin. I make iced bulletproof-inspired coffee with mct oil, heavy whipping cream, salt, and toffee stevia blended up together. For hot coffee, butter and/or coconut oil blended up with the coffee are popular choices.