Trump's biggest lie

The Washington Post is keeping track of Trump’s lies.  As of the 10th of this month he had told 6420 lies.  That is truly amazing.  How do I decide the biggest one?  Technically he wasn’t president until the end of it, but here it is, right out of Article II, Section 1, of the United States Constitution:

“I do solemnly swear that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.”