Purple sweet potatoes and blueberries is really good, I add a bit of stevia and some coconut milk.
My very favorite has dark cocoa, frozen raspberries, stevia and sugar-free almond milk. It is so rich-tasting it seems decadent but every ingredient is healthy.
Strawberry or strawberry banana. I use frozen fruit...and I don't know if it's technically a smoothie anymore when I get done with it. More like a thick shake. I add sugar free sweetener, powdered coffee creamer, (which acts as a thickener), and chocolate protein powder.