I really really love using Forager Project cashew yogurt in this watermelon yogurt smoothie. At this point, I pretty much switch between that and unsweetened coconut yogurt (which I used in my Piña Colada Smoothie). However, any yogurt will work. If you're okay with the smoothie being vegetarian and not vegan, Greek yogurt is a great option. A sweetened or flavored plant-based yogurt is also an option, though I highly recommend sticking to vanilla or strawberry so the flavor of the fruit still shines through. Lastly, if you're okay with a thinner smoothie, plant-based milk should work as a 1:1 substitution too.