Swallowing your own tongue (like Miggs from silence of the lambs). I dont know why, that always seemed really creepy to me.
Death on a cross. Not to bring religion into this, but that is one of the most horrific, torturous deaths mankind has invented/used regularly.
The funny thing about crucifixion is that the most horrible and evil criminals were crucified upside-down. While considered the most dishonorable way to die, it was actually more humane, in that death came much faster.
Of those listed, tortured to death. They can make you last a long time that way. But of those not listed, crucifixation is proably one of the worst, time wise. People have lasted up to three days, before dieing on the cross. And here is another one that is just as bad, timewise. Have a blunt stake hammered up your anus, then set out to die somewhere. Some people have been known to last three days, before the blunt peg worked its way up their body, and it struck some vital organ, causing death.