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In some countries, the head of state can pardon anyone for alleged crimes. For example, in Nazi Germany, Adolf Hitler pardoned murderers of communists and Soviet prisoners of war, in those cases in which those crimes were actually prosecuted and resulting in a conviction.

In the U.S.A., the President can also pardon anyone for (alleged) federal crimes. What checks and balances exist to prevent a U.S. President from pardoning anyone convicted for a federal crime when he or she thinks that shouldn't be a crime in the first place (apart from losing re-election if such pardons are impopular)? Reportedly, President Trump has said has said that officials who take private land in order to build a border wall will be pardoned (a White House official has reportedly said this was a joke).

Given that "an impeachable offense is whatever half of the House of Representatives considers it to be", impeachment could be a possible remedy if a President's actions were flagrantly immoral and/or sufficiently unpopular. The Constitution says that a president can be impeached and removed if they commit "high crimes or misdemeanors", but this term is not defined elsewhere in the constitution, and was understood by (at least some of) the Founders to be a catch-all for any betrayal of the public trust.

Contemporaneous comments on the scope of impeachment are persuasive as to the intention of the framers. In Federalist No. 65, Alexander Hamilton described the subject of impeachment as

those offences which proceed from the misconduct of public men, or, in other words, from the abuse or violation of some public trust. They are of a nature which may with peculiar propriety be denominated POLITICAL, as they relate chiefly to injuries done immediately to the society itself.

In fact, the Founders in fact explicitly considered impeachment to be a remedy for abuse of the pardon power. The possibility of abuse of the pardon power came up during the debates on the Constitution, with George Mason arguing that the President should not have a pardon power lest he pardon those he had conspired with to commit a crime. James Madison pointed out that impeachment would be a valid remedy in such cases. [Thanks to @BradC for providing this link in the comments.]

Theoretically, the congress could try to impeach.

BUT, in reality, presidents often wait until his final days/hours to grant the most controversial pardons.

And once a pardon is granted, there is no recall of the pardon.

Therefore, there is really no checks to pardon power, making it the most KING like power the presidency holds.