Of all the times in my life I've seen someone tickled or tickled someone it's nearly always finger or fingernails. I guess you can include tongue for ticklish foot worship situations, but it wouldn't be the technique that tickles my wife the most. For my wife, I think the long manicured nails of her female friend(s) works well. However there have been two times where I used a tool to tickle her feet: 1) a guitar pick, which was laying on the nightstand after I emptied my pockets at the end of the day and 2) a stiff bristled brush.
The guitar pick incident was fairly long for us. I never tickle her for more than a few minutes, and this lasted maybe 2 minutes. It drove her crazy. She kept asking while laughing "what is that?!". The brush incident happened because her feet were covered in some kind of oil after a massage that she had and she couldn't get it off. I saw her scrubbing her feet with it and went in and told her I'd help. Her feet were wet and I put some soap on the brush and started scrubbing her soles. She held out for a bit but started laughing and squirming until finally she said "ok ok! no more! I'm too ticklish!"
In my limited experience, I think the long fingernails tickle her the most.