My feet are SUPER ticklish. But the the tool and technique that makes me want to crawl out of my skin is actually a cheap, Walmart brand soap saver. It has an elliptical shape and it's white/semi-transprent. It's flexible with these little rubber "nodes" on it. When my feet are lotioned or oiled and that thing is scrubbed back and forth across the soft pad of the ball of my foot, I will wail and laugh so hard it's silent. I actually broke an under the bed restraint once from trying so hard to pull away from this. I've never been in stocks and toe tied before but it would be the absolute height of ticklish torture if the same thing was used while restrained like that. A very close second is a Phillips Sonicare toothbrush. Now the bristles don't tickle me nearly as much as the back end of the brush head for some reason. And that thing is pure torment on my toes. The base of my toes, the pads, in-between them...I laugh so hard I get light headed. I had a tickler take the Sonicare brush and she isolated each individual toe and spent time casually stroking the back end of the brush head up and down each toe. From the tender base to the plush sensitive pad of the toe. I would just lay there helplessly losing my mind while she spent minutes on each toe back and forth. Each time she completed all ten I would think it was over and then she'd oil them and go again. And each time she'd give me a moment of false hope like it was the last time and then she'd go again.
So yep! Those are my top 2 tools. Hairbrushes are too harsh because my feet are very soft and sensitive. But a silicone scalp brush is almost equally as tickly as the soap saver. Feathers only tease my feet. Fingers are always enough to completely tickle torture me though. Nothing beats the classic skin to skin contact of a tickler wrecking you with fingers.