I vote feet because of 3 reasons. Reason 1, they seem more easier to access and harder to defend than armpits. To get someone's feet, all you need is get them in your grab. To properly get armpits, you need pretty solid bondage or holding technique because unless arms are stretched above the head, there is no proper access to armpits in my opinion. I mean, there is, but it is nowhere as ticklish as when arms are stretched. So, feet are easily attacked without bondage while armpits are not, unless you have someone holding victim for you or you tie her down properly.
If I ignore that access thing and assume I get good bondage etc, then I still vote feet as generally more ticklish spot than armpits, but that is just a guess since there are people who are more ticklish on armpits. It is kinda individual, but if I have to guess ticklishness of some random person, I'd vote to get her feet first as potential best spot.
And for a third reason, it seems to me there are more tickling techniques available to get someone's feet, and that makes it more fun. Everything that works on armpits generally works on feet, but not everything that works on feet work on armpits (forks etc). At least I'm not aware what can work on armpits and not on feet assuming someone is ticklish on both spots.