Having read a few books about this many years ago I came to the conclusion that two forms of torture are particularly intriguing.
A favorite of Latin American fascist governments of the 70's was El Submarino. Hang the victim by his feet and submerge his head in a bucket of urine & feces. Do this every morning before breakfast.
Another favorite is a drug administered to some prisoners of the Soviet Gulag. The drug made the victim too tired to stand and gave him vertigo when he lay down.
Prisoners can survive months of these tortures without permanent physical harm.
Wasn't there a scene in an old Woody Allen movie (from when he was still funny) where he was tortured by being locked in a room with an insurance salesman? Other survivable but horrible methods of torture:
