Re: Which jutsu would cause the slowest and most painful death?
Well, one would probably be Hidan's curse.
A second would be gentle fist (strike the right way, and you can cause slow internal bleeding/organ failure) which could be very painful and long.
Then pending speed, Shino's parasite
If he cut you right, Kiba's Tail Chasers Spinning fang would probably leave you with multiple missing limbs and bleeding to death.
Mokuton/Doton, could slowly crush you to death
Controlling the spread of the flame to a minimum, Sasuke's Enton would be agonizing
I'll stop there
Edit: Ah...Sasori's poison, and getting your heart torn out slowly by Kakuzu....forgot about those....don't know how. Guess you could add Hanzo's poison.