I posted a thread about this a while ago. My wife can be a very determined and tough to break lee despite being extremely ticklish. We bought this pink rubber/silicone scalp massager at Home Goods one time and the second I saw it I thought of tickling her with it and got turned on. Needless to say, when we were home later, she let me try it on her feet and she went crazy in an instant. She yanked her feet away and started saying "no no! Not that! Its too ticklish!" And she went ballistic as I tickled her feet with it.
In more recent sessions it has been less effective on her, disappointingly. Though I do find it works much better after the application of oil/lotion to the body. Also seems like mostly a foot tickling tool to me. Can't see myself tickling her upper body with it, though I considered it. I just dont see it being as effective.
Obviously the metal ones posted in the image above aren't going to work. Nothing personal, but they dont even look like tickling tools. You have to get something similar to the below image. My wife described it as feeling like a lot of tiny fingers tickling her or something. It also works better when the feet are stretched taut so you can scrub/massage properly.
Obviously, no tickling tool is completely perfect in all situations but this one is definitely a great tool to add to the collection.