Probably fingers. There are a lot of clips where you see some guy drilling a woman's belly button with his fingertip like he's putting up some shelves which she then pretends is the funniest thing ever. In my experience with the women who have been good enough to let me tickle their belly buttons (ranging from not that sensitive to off the charts ticklish) I had the best response from very lightly tickling, almost brushing with my fingertips where I have fine control over the level of pressure.
Of course a good raspberry/zerbert will usually get a laugh and gently blowing and nibbling can get nice reactions if things are going in that direction.