I've used an egg vibrator with excellent results. I pull the string between her toes until the egg rests against the underside. When I turn it on its like electric shocks are going through her and the screams are intense.
Fingers are and will always be the most versatile tickle tool. They can be soft or firm, slow or fast, gentle or digged in, and can be used everywhere that is ticklish.
Having said that, a blush brush (makeup brush) is an amazing tool for specific areas.
For feet.
1) Sharp fingernails (always the best)
2) tongue (combined with fingernails and nibbling it can be a really torture for some people who, mostly for those that think feet are gross or for people that became very sexually aroused with this fantastic technique))
3) teeth (very nice to nibble and in cases of unticklish feet biting to produce some nice pain!)
4) Brush (I am not an expert using it and I see many videos where the brush is applied to produce a strong reaction, mostly by the pan it produces so for foot torture it could be nice)
Armpits ribcage thighs
1) fingers
2) fingers
3) fingers (although lickling armpits can be effective
Skin in general (fingernails, fingertips and tongue)