Being a foot guy, I used to tickle my former girlfriend's feet all the time, with little or no reaction really.
Then one time while we were at the Fetish Club where she worked, I snuck up behind her & just 'placed' my fingers under arms. She locked my fingers in & fell to the floor in a fetal position screaming & squirming violently.
I was totally SHJOCKED by this because she usually NEVER showed any vunerabilites to my ticklings.
Oh, she was PISSED off at me, yelling at me, "DON'T YOU EVER TICKLE UNDER MY ARMS AGAIN IN PU8LIC."
After a couple of days we were in bed & I said, "Let's play a game. Let me tickle your arm pits & see how long you can last before you get mad."
Incredibaly she agreed, since she never backed out of a challenge.
She lay on her back & lifted her arms over & behind her head. She held on the headboard for dear life, & clenched her eyes shut, anticipating the torture.
I slowwwly stroked her armpits for about 3 minutes until she screamed at me , "NO MORE! HAHAHAHA! FUCK! I CAN"T . . . HAHAHAHAHA! STOP  NOW!"
The entire time she was squirming & giggling uncontrollably.
That was the ONLY time she let me do that.
But I have to say that it was so VERY erotic to me that I was finally able to get a REAL ticklish reaction from her.  
Now, just because I am a foot guy doesn't mean that's ALL I will tickle (And lick). It's just my main passion.