The I'd have to choose thumbscrews, since all those other would take awhile. Racking would be a close 2nd. Pins under the fingernails works well, also; and removing the fingernails with pliers is a backup plan for those who insist on being difficult to work with.
Now, if I had plenty of time to get the answers, or wanted to punish as well as kill, I'd go with wheeling (which you didn't offer as a choice).
Another one, which I'm rather fond of, is using a rope, along which knots have been tied. The rope is placed around the head (just above the ears) and cinched. Slowly tightening it causes the knots to press into the skull, possibly cracking it at the contact points. After awhile, the skull will be crushed from the stress, but the "witness" will offer a great deal of valuable information (some of which may even be true) in the meantime.