Obvious answer for a thorough relentless tickling would be my armpits, and perhaps bellybutton (what comes to mind is that delicious Umojar artwork of Jaynin's Felin tickling Haruhi navel with a tiny vibrating magic wand).
But to offer the more horrific scenario of torture, I'd have to go with the cock and balls. Imagine being stripped naked and tightly restrained in a way that no escape is guaranteed, and your captor is slowly, methodically working your cock over with a stiff feather. It's not a hysterics-inducing tickle by any means, but it ever-so-slowly works you up to the brink of orgasm-- when suddenly said feather flicks downward and swirls cruelly along your ballsac, driving the tickling sensation through the roof and killing that hope of orgasm in an instant. Then after several agonizing minutes of this, the feather slides back up to work the cock. Rinse and repeat.
After a while, your broken pride will give way to pitiful begging for orgasmic release. Several more hours of that, then you talk. Interrogation successful.
