They're portable, nutritious, and you can change the taste based on what you like and what you have on hand.
In the beginning, I thought it was better to put as many different kinds of fruits and whatever as possible.
But I find that smoothies taste better if there is a base plus one or two complimentary flavors (like pineapple and banana or apple and peanut butter).
My base usually includes: yogurt, a banana (because they make a smoothie extra creamy), and ice.
My usual throw ins include: fruit,spinach, peanut butter, coconut oil, cocoa powder, orange juice, oatmeal/granola, and/or brewed coffee.
I never put all those in at once, just one or two. :)...... or three.
Here are a few of my favorite combos (and I never measure. I just throw in a portion that looks good to me):
1. Green Tropics Smoothie
1/2 C. (?) pineapple
handful of spinache
yogurt (plain or vanilla, w/e I have)
spoonful of coconut oil
2. Banana Nut Winter Smoothie
handful of chopped walnuts (or whatever nuts you have)
3. Chocolate Protein Smoothie
One last tip:
I like to buy canned fruit (packed in juice, not syrup) such as pineapple in the largest can I can find because it's cheap and already cut up. When I open the can I'll put in only the amount I want, then freeze the rest in an ice cube tray. Once frozen, the cubes can be stored in the freezer in a container. The next time I make a smoothie, I'll pop 2 or 3 frozen pineapple cubes in there. It also does double duty as the ice factor.