I think that, if your torture will be aired on TV and should act as a deterrent, it can't be 3 hours of screams and agony. If one day something like that happens on TV, most people will find it revolting and with time even boring. You need something entertaining if you want people to watch it.
That's why I think that josh-king answer, while giving the maximum amount of pain possible, is not fit for TV.
However what you can imagine, to continue with josh-king answer, is a wireless device surgically connected to the brain's pain center. This device would be controlled remotely to provoke pain.
I don't have any knowledge in biology so I don't know exactly what power (volt and amps) is needed to give your subject a strong stimuli, but since your talking about nanorobots, I think that with some research and a few
prisoners volunteer test subjects, the level of technology in your world is high enough to produce such device.
Once your subject (or subjects) has the device implanted, all you need is to create a sadistic game. Disguising the torture as a game will make it entertaining to your viewers will instilling fear in their mind. And with enough propaganda, your dictator can even be seen as a good guy, giving a last chance of redemption to traitors.
Why don't you create a reality TV show ?
You take like 10 prisoners and you locked them up in a manor. In the beginning, the prisoners have nothing (no food, only a few cloth...) and are locked in the main room.
While being in the main room, they feel a mild-strong pain (something equivalent to a toothache).
Every day at the hour of your choosing, a door open. At this point you want to increase the pain level of everyone inside the main room because you want them to enter in the newly opened room.
Inside it, you will have your brand new torture of the day, a sadistic challenge that the prisoners should face to gain something.
You can make them gain food, clothes, even painkiller that'll just be placebo because you're in control of the pain. You can even try to make them betray each other, pledge unending loyalty to you, beg for forgiveness in exchange of personal rewards, whatever fits your need.
The torture of the day shouldn't necessarily be something painful, but something that looks painful, something that the viewers would love to watch. Also, you should design every challenge so that submission to your dictator's and the value he worships are rewarded. It would "educate" your viewers while brainwashing the prisoners.
For example, you can imagine the following challenge. Your prisoners are asked a question with two answer possible. They can choose between betraying their country or putting the supreme leader's life at risk. They have a limited time to answer the question and the more time they take to answer the question, the stronger the pain they'll fell, pressuring them to answer just to stop the pain. Obviously the only right answer is silence. Anyone who answers will be led to a room of suffering (maybe even death) while someone who remain silent until the end to the timer, enduring the increasing pain, will gain something. This way, you're both teaching your spectators that they should not betray your dictator or the country and you're making the prisoners suffer.
Moreover, while inside this manor, your prisoners need to have a goal. An obvious one would be to collect every piece of a key through every day's challenge. This key would then open a big door to freedom (or that what they're told). Without this hope, most of prisoners will want to end their suffering at some point. And even with this hope, it's possible that some of them will want to commit suicide. Be sure to put something to help them.
Even with a way out, most prisoners (maybe even all of them) will die before completing the key, but that's not a problem because you just want them to suffer.
If someone manages to obtain every piece of the key, you have two choices :
You can set him free if he's been obedient and the dictator is feeling generous.
Or you can make him face one final challenge while enduring excruciating pain, with death as the only possible outcome.
With this solution the population will see the traitors suffer, but it'll be entertaining (somewhat). You could run this kind of show for weeks, even month. With enough dissidents, you could even have a new edition each year !