I want to go with invisibility, except part of what I get out of tickling is the joy of watching them react to my physical presence, which is considerable, so in terms of superpowers, I'm going to go with Four Arms. I'd keep the spares folded and tucked away most of the time until it was Tickle Time and then FOOP! Four big muscular arms, four meaty hands, 20 wriggling fingers and nowhere is safe 😀
And yes, that does mean double the armpits so when I met my nemesis -- any of you, it sounds like -- I would have that additional weakness as well, yes. 🙂