I'd like to add my vote for the electric toothbrush having been on the receiving end. A washing up brush is good too, but it can be painful which isn't for everyone, but is definitely OK for some
I've only ever tickled or been tickled by a simple sort of make-up brush, maybe an inch in diameter and fairly soft with shortish bristles, but it was definitely great at producing constant giggles, nothing intense but then it doesn't always have to be intense. 🙂
The elecrtic toothbrush (a used brush is softer) drives me to breaking point almost immediatly! I acually had it used on me yeasterday and my throat was sooo sore today from screaming! It can be used anywhere that she is ticklish and even places you dont get much of a reaction will have her begging for you to stop! Feet and toes are the worst for me but behind the knee and bellybutton were ranked right up there on the scream-O-meter. Use a light touch though. To hard can kinda hurt a bit so take it easy and get ready for her to beg!