I guess it would be cheating to say all of them?
As one who believes there are a million types of tickles and a million ways to give them, this is almost an unfair question. So many reactions, so few words and so few languages to articklelate. *grin*
One reaction I love is "the resistance". When a loud laugh, squeal or spasmodic movement would not be appropriate, yet a dastardly sneaky tickle has been initiated. Soft gasps are made, threatening glares are unleashed, lips are bitten down on and on lucky occasions, mouths are widened, eyes are clenched shut and heads are thrown back in silent hysteria.
Next, the "break up". The resolute stoic facial expression dissolves feature by feature into it's own visual expression of ticklishness. It's like the ice ona frozen lake melting bit by bit in spring.
Another adored reaction is the very first muscle twitch. Doesn't matter which muscle, nor whether it's accompanied by an audible sound. I'd be willing to bet almost anything, that initial spasm, regardless of size or age, rivals any professional athlete's fast twitch muscle response.
I love ANY surprised expression. "I can't believe you found THAT spot!"... "I can't believe how much that tickles!"... "I can't believe you REMEMBER that spot!".... "I never KNEW about that spot!"
..etc.
Of course, the laughter, the Laughter, the LAUGHTER!! But especially that mysterious laughter which has endless resources. The laughter with a mind and destiny of it's own. The laughter that reaches it's limit, then decides all on it's own to expand it's limits! Mercy!
Then there's the reaction that tells you in so many words "This no longer JUST tickles!"... and this reaction can be verbal or non-verbal *wink wink*
Ok and for my favorite... I can't distinguish, so it's a tie.
It's the truth. I never tire of the truth. To me, when there's ticklishness...and there's tickling.... the result is the truth. (Consent goes without saying in my book).
and
It's the radiance afterwards, the glow on her face that tells me she's been exactly where she craved/wanted/needed to go and is now ready for a return trip!! And then MY reaction requires oxygen!