Certainly a contender, along with:
Tokyo, May 2019 (Trump).
North Korean state media had called Biden a fool of "low IQ". Trump tweeted from Japan that he "smiled" when Kim did so. At a press conference beside Prime Minister Abe, he said he agreed with Kim's assessment. This is arguably the closest match to the Xi episode: siding with a hostile authoritarian regime's insult of a domestic political rival, while on foreign soil. It is also by the same president.
Helsinki, July 2018 (Trump again).
Standing next to Putin, Trump was asked about Russian interference in the 2016 election. He said Putin had been "extremely strong and powerful in his denial" and declined to back his own intelligence agencies' finding. He walked the remarks back the following day. Many Republicans at the time, including John McCain, condemned it in the strongest terms. McCain called it "one of the most disgraceful performances by an American president in memory."
Hanoi, February 2019 (yep, it's Trump).
Trump said he would take Kim Jong Un "at his word" that he didn't know about Otto Warmbier's treatment. Warmbier was the American student who died after detention in North Korea.
Oval Office, May 2017 (fucking Trump again).
Trump once again. This time he shared highly classified intelligence, reportedly supplied by Israel, with Russia's foreign minister and ambassador. The Washington Post reported it, and the administration did not dispute that the information was shared.
Alien and Sedition Acts, 1798 (John Adams).
These criminalised "false, scandalous and malicious" writing against the government and were used to jail newspaper editors and a sitting congressman, Matthew Lyon.
Indian Removal Act, 1830 (Jackson), and the Trail of Tears that followed.
Jackson's famous line "John Marshall has made his decision, now let him enforce it", about defying the Supreme Court, is probably apocryphal.
Executive Order 9066, 1942 (FDR).
This led to the internment of around 120,000 people of Japanese ancestry, most of them US citizens. The US government formally apologised in 1988.
Wilson, 1913 onwards.
His administration resegregated the federal civil service. The Sedition Act of 1918 was used to imprison critics of the war, including Eugene Debs.
Watergate (Nixon) and Iran-Contra (Reagan's administration).
Iran-Contra involved secret arms sales to Iran with proceeds diverted to the Contras, in breach of the Boland Amendment.
January 6, 2021 (Trump. Again).
For many historians this is the leading modern contender, since it involved the peaceful transfer of power. Trump was impeached for "incitement of insurrection" but acquitted by the Senate.
Let's be very clear on this one point.
The President mocking a former President in front of the communist leader of China is the most un-American thing an American president has ever done.