I use a yellow pea protein in my smoothies and fruit juices often mixed with ice water. And a fruit - most often cherries.

Tofu works really well in smoothies if you're trying to avoid dairy. We use it when we have leftover bits that didn't get used for something else.

Banana/Mango works well with any liquid to give the smoothie creaminess, then coconut milk is a favorite of mine, for the flavor and the richness. I tend to use cashews ( not even soaked) or oats as well, chia seeds, avocado, frozen pineapple, frozen melon. All for now :)

I mostly use milk or almond milk in my smoothies but found that Cran Apple or Cran Pomegranate is really tasty too :)

I echo the almond milk route.

Soaked cashews will make your smoothie creamy with no milk needed. This is also a great trick for making creamy salad dressings from scratch in your blender.

If you are into yogurt, you can try to find So Delicious coconut milk yogurt. It is delicious. Or, if you are more DIY, you can make coconut yogurt yourself with kefir starter and coconut milk. It is surprisingly easy. You just warm up the milk a little, add the packet (its like yeast) and leave it unattended overnight.

Water and fruit works fine.

Coconut milk is excellent.

O'Soy makes a great soy yogurt; my son eats a ton of it since he has a dairy allergies. I make him smoothies with it all the time.

Almond milk is also excellent and pairs really well with banana as a thickener. Avocado is another creamy addition that sneaks well into smoothies!

Too many choices to list them all but I like Soy Milk personally.
Take a look at Foodily: http://www.foodily.com/s/smoothies-without-dairy

You can use juices, soy milk, almond milk, etc. Then use fruit like banana for thickener. There are also soy based yogurts. Enjoy!