Depends on the 'lee, and the situation. When it's playful/part of foreplay, fingers and tongues; when it's playful torture, feathers, paintbrushes, or flossers.
It's more about the touch than the tool. If a lee has a ticklish belly button, it seems a more sensitive, light touch works best. Poking doesn't work so I'm always doubtful when I see that in clips. Stroking and brushing with the fingertips, raspberries and nibbles seem to work best. They also support the old adage that it's better to give than to receive.