I actually start with the neck if I can, it really depends. If it's a friend sometimes I'll tickle the ribs or feet if I can get them. It's hard for me to tickle at times, I don't want to miss and hit a boob or something with a girl friend, most of my friends are female, always have been. Most don't like being tickled either, but if I get the chance I'll do it, ribs and neck usually. I always like to feel her head tilt to the side and try and squeeze my fingers (I have long thin fast piano/braille fingers) and of course that just traps my hand within the hallow by the collarbone, you know the place.
Foot tickling as a first spot is good if I am giving a massage, but since many of the friends I have don't like tickling anyway I only do it for a few seconds. I have not had the pleasure of a long term tickling session but some day it will happen. That was perhaps much more detail than you wanted to know, but I thought I would write it all out for clarification rather than just a certain spot.
dave