Politifact has named President Trump's ongoing denial of Russian election interference 2017's Lie of the Year:
Trump continually asserts that Russia’s meddling in the 2016 election is fake news, a hoax or a made-up story, even though there is widespread, bipartisan evidence to the contrary.
When the nation’s commander-in-chief refuses to acknowledge a threat to U.S. democracy, it makes it all the more difficult to address the problem. For this reason, we name Trump’s claim that the Russia interference is a hoax as our Lie of the Year for 2017.
Readers of PolitiFact also chose the claim as the year's most significant falsehood by an overwhelming margin.
Full story: 2017 Lie of the Year: Russian election interference is a 'made-up story' (Politifact)