1) Feet (especially when I have them in my stocks with unrestricted access to the entire foot)
2) Breasts and nipples (double bonus, as it usually is a combination of ticklishness and sexual stimulation). I had one ex-gf tell me afterwards that she was thinking (to the extent she COULD think while it was happening) "What in the world is this guy doing to me?!" She couldn't wait to try it again. 😊
3) Armpits
With my current GF, her armpits are the Holy Grail. She wants me to tickle them but she is so ticklish there that she has to protect them at all costs. A big part of our playtime is me scheming up new ways to get at her armpits and nipples and her trying to resist. I love tickling her neck as well because that really gets her going "down there".
Outside the bedroom, I'm reaching over to tickle her inner thighs while were stopped in traffic or at a traffic light. Sometimes, we'll get to our destination and she will suddenly put her foot in my lap and say "tickle me". I had to bring her along slowly at first, but she's really getting into it now. 😁