I have to go with fingers. I love to feel the skin of the feet and toes and their wiggling when I tickle them. Nothing wrong with a feather, but it's just not the same. I wouldn't use my tongue unless I were with my wife or girlfriend (not attached at the moment). And an electric toothbrush? Different, but not satisfying. Pretty long reply for a poll, but hey, we are talking about tickling feet here! 😀