So far, kale has been my favorite green to put in green smoothies (no surprise there), but I've used spinach and chard too. Spinach has a more assertive taste, and it's been a bit overwhelming in a couple of my smoothies. I found that pineapple stands up to spinach pretty well, so that's been my spinach smoothie strategy. You still get some nice spinach notes without feeling like you're drinking a spinach salad. And the color is gorgeous to boot.
1/2 cup orange or pineapple juice
3/4 cup frozen pinapple
Big handful spinach (maybe about 1/2 cup?)
1 tbsp ground flax seed