Very hard to pick one, but all things considered, I'll have to go with Tickle Torture.
Was one of the first studios I ever came across, and ever purchased from. Lots of great models have come through there, and they usually make sure that they have something for everyone when it comes to tickle spots.
They haven't been updating a lot lately, so I don't know if they're moving on from tickling, but their stuff always was great.