As expected, Trump's pants are on fire big time. 60.13% of his statements have been false, 2.53% true. Throughout his entire campaign! Hillary ranks 13.33% and 13.33%, respectively.
Looking beyond the two nominees, Source two.
And Source three.
As for the question if the current two candidates are the biggest liars ever to run for president, here's an interesting read. I think the most important part is the very last paragraph:
"Telling the truth matters, even in politics. But we should remember that today, as at other points in our past, charges of lying often arise not out of sober concern for the sanctity of our public discourse, but as a way to score quick and wounding points in the partisan joust that is American democracy."