Baby spinach works best: the leaves are more tender than mature spinach and bring less bitterness. Frozen spinach (thawed and well drained) also works — it even gives the smoothie a cooler consistency. The stems of baby spinach can go into the blender too; they are mild enough.
The sweetness adjusts well: a ripe, soft banana is essential — an unripe banana brings less sweetness and a slightly starchy note. Mango balances the grassy-fresh note of the spinach especially well; if you do not have mango, you can use a ripe pear. Lemon juice is optional, but it brightens the color and rounds out the flavor.
Mind the order in the container: water first, then spinach, then fruit on top. This order protects the blades and starts the vortex cleanly. For small amounts, use the tamper — spinach leaves can pile up at first.
If you blend green smoothies often, you should rotate the greens: spinach every day as the only source of greens is less advisable because of its oxalic acid content — alternating with lamb's lettuce, kale, or cucumber is the simple addition.