What are you referring to exactly?
Thank you for asking!
The lie that leaving the EU would free up £350M of public spending that could be spent elsewhere is a big one, obviously.
But Leave was also claiming we'd have tariff-free access to the single market without the same obligations that members have, as per this BBC article from before the referendum: www.bbc.co.uk/news/uk-politics-eu-referendum-36249625
""The UK, as the second largest economy in the EU and the biggest export market for the rest of the EU, we will have tariff-free access to the rest of the EU when we vote to leave. I believe that will happen." " - Matthew Elliot, Chief Executive of Vote Leave.
Long way away from the WTO rules now being bandied about, eh?
Goodness me, if I hadn't paid much attention to the EU other than rolling my eyes at silly b-EU-rocrat stories and then someone told me we could have our cake and eat it, I would probably have voted for that too!
And given how close the vote was, this stuff made a difference, as Dominick Cummings himself acknowledged: "If Boris, Gove, and Gisela had not supported us and picked up the baseball bat marked ‘Turkey/NHS/£350 million’ with five weeks to go, then 650,000 votes might have been lost."
blogs.spectator.co.uk/2017/01/dominic-cummings-brexit-referendum-won/
And of course if we widen to lies in general rather than just lies about the benefits of leaving the EU (because let's face it, Leave had its own Project Fear going on too), there is the assertion that "Albania, Macedonia, Montenegro, Serbia and Turkey are joining the EU" Not "are in the process of joining but can be stopped by a veto", but a flat "Are". You can see that ad along with the rest of Vote Leave's targeted Facebook ads in this submission to a committee of MPs investigating fake news
www.parliament.uk/documents/commons-committees/culture-media-and-sport/Fake_news_evidence/Vote-Leave-50-Million-Ads.pdf
(The £350M is there as well. Lots of times!)