One of my favorite responses when tickling a woman is when she simply says "I can't" and I usually response "can't what?". Of course they are usually too preoccupied or exhausted from laughter to respond. Another favorite of mine is when they say "no!" while laughing, also very satisfying. And...
The worst that's happened to me was the cousin of a one of my friends wasn't AS ticklish as she was. My friend was very ticklish everywhere and simple using my index fingers I could tickle her sides to cause her to squirm and tilt her head back. Another time she told me how she hurt her ankle so...