Each of these characters fails the final test. All of them make mistakes, either over-estimating themselves or underestimating others. Selecting the most shrewd from this list, I'd say Huck qualifies because he does not get tarred and feathered. The King and the Duke are savvy and tricky, but not shrewd enough to know when to move on.
The Duke is shrewd in that he is a conman. Huck does not think like a conman. He generally is somewhat naive, despite his life experiences, and still expects the best of people. However Huck is also very intelligent, and possibly moreso than the Duke. He definitely has a higher moral code.
While the King and the Duke are very conniving and cunning, their cupidity causes them to ignore the practical. For instance, when they know that people are beginning to become suspicious, instead of taking flight, they remain because of their greed for money and because they delude themselves into thinking that they can outsmart people. Huck, on the other hand, retains the innocence of childhood which prevents him from the deadly sins of the King and the Duke which cloud their judgments.
"Shrewd" is defined as "characterized by keen awareness, sharp intelligence, and often a sense of the practical." In The Adventures of Huckleberry Finn, Huck shows these characteristics of shrewdness more often than the king or the duke.
Huck and Jim listen to the life histories presented by the king and the duke with rapt attention and are happy to play the roles of respectful subjects by addressing them with their royal titles, serving them their meals with due decorum, and giving them the best areas on the raft for their beds. However, Huck realizes the truth of their situation quickly.
It didn't take me long to make up my mind that these liars warn't no kings nor dukes, at all, but just low-down humbugs and frauds. But I never said nothing, never lot on' kept it to myself' it's the best way' then you don't have no quarrels, and don't get into no trouble.
Huck remains practical throughout the time that the king and duke are in his life. He knows they aren't what they say they are, but understands it's best to humor them. He understands that Jim wouldn't be able to comprehend the true situation, so doesn't go into details with him.
What was the use to tell Jim these warn't real kings and dukes? It wouldn't a done no good; and besides, it was just as I said; you couldn't tell them from the real kind.