It's less a real answerable riddle, and more a thought experiment on the nature of power.

I already answered this question on Puzzling, but unfortunately the question there was deleted. Here, though, is the answer:

“The king, the priest, the rich man - who lives and who dies? Who will the swordsman obey? It's a riddle without an answer, or rather, too many answers. All depends on the man with the sword.'

'And yet he is no one,' Varys said. 'He has neither crown nor gold nor favor of the gods, only a piece of pointed steel.'

'That piece of steel is the power of life and death.'

'Just so ... yet if it is the swordsmen who rule us in truth, why we pretend our kings hold the power? Why should a strong man with a sword ever obey a child king like Joffrey, or a wine-sodden oaf like his father?'

'Because these child kings and druken oafs can call other strong men, with other swords.'

'Then these other swordsmen have the true power. Or do they? Whence came their swords? Why do they obey?' Varys smiled.

 

[...]

 

Tyrion cocked his head sideways. 'Did you mean to answer your damned riddle, or only to make my head ache worse?'

Varys smiled. 'Here, then. Power resides where men believe it resides. No more no less.'

 

_{-- A Clash of Kings, emphasis mine}

It's less a real answerable riddle, and more a thought experiment on the nature of power.

I already answered this question on Puzzling, but unfortunately the question there was deleted. Here, though, is the answer:

“The king, the priest, the rich man - who lives and who dies? Who will the swordsman obey? It's a riddle without an answer, or rather, too many answers. All depends on the man with the sword.'

'And yet he is no one,' Varys said. 'He has neither crown nor gold nor favor of the gods, only a piece of pointed steel.'

'That piece of steel is the power of life and death.'

'Just so ... yet if it is the swordsmen who rule us in truth, why we pretend our kings hold the power? Why should a strong man with a sword ever obey a child king like Joffrey, or a wine-sodden oaf like his father?'

'Because these child kings and druken oafs can call other strong men, with other swords.'

'Then these other swordsmen have the true power. Or do they? Whence came their swords? Why do they obey?' Varys smiled.

[...]

Tyrion cocked his head sideways. 'Did you mean to answer your damned riddle, or only to make my head ache worse?'

Varys smiled. 'Here, then. Power resides where men believe it resides. No more no less.'

_{-- A Clash of Kings, emphasis mine}