Americano
An Americano is simply an espresso with hot water added on top, containing only 1 to 2 grams of carbs depending on the size (or shots of espresso in the drink). If you've never had one, drip coffee and an Americano are essentially the same in terms of flavor. According to Spoon University, the typical cup of coffee is brewed at a lower temperature for a longer period of time, which helps to reduce the bitterness of the beans. Nonetheless, Starbucks' dark roast is known for being fairly powerful, so the flavor change is minor.
Your Americano will have more or less caffeine than a typical drip coffee depending on how many espressos are in it (more if you buy a venti). Because the drink is simple and straightforward, feel free to customize it with keto-friendly additions like heavy cream, stevia, monk fruit sweetener, or cinnamon.