Jeez! I don't know if I could pick just one spot. Your girlfriend has some hot feet. I love how her second toe is just a little longer than her big toe.
Lemme see ........ it's a debate between the two 'toe' spots & the arch ...... um , I guess I'll pick the arch 'cause that area might also include the balls of the feet , giving me more to play with ....... but those toes are so luscious I'd have a difficult time leaving them alone.
If I had to guess , I'd say that your girlfriend's most ticklish spot on her foot would be 'under the toes'.