I use a combination of things - it varies - yoghurt, milk, avacado, berries & honey/another sweetner makes a nice shake.
yoghurt, berries/fruit, ground nuts & seeds, ground oats sweetner/honey, plus milk to thin as needed (make thinner than you expect because oat absorb liquid and will soon thicken the shake is another I use from time to time. No real set quantities, but mix and match as needed - can leave the yoghurt out if you don't have any etc. Avocado makes the shake silky smooth. Oats make it thicker (use a coffee grinder to mill them). Ground nuts & seeds add protein, fruit & sweetner add the flavour. I say sweetner because I use agave nectar rather than honey, you could also use maple syrup, golden syrup or just straight sugar if needed.