Trump is one of the most ethically-challenged "politicians" around. His unscrupulous business practices are well-documented ("New bio 'Trump Revealed' offers troubling portrait"), and commentators on both the left and right have highlighted how Trump repeatedly contradicts previous statements and then denies what he said initially. See, for example, the conservative website Red State's article on Trump's claim that he was always against the Iraq War inspite of evidence to the contrary (Trump Repeats The Lie That He Was Opposed To The Iraq War “From The Beginning”). Or check out Stephen Colbert's "Trump vs. Trump" debate ("which was held" back in January):
Bottom line: Trump may claim he always tell the truth (he says that so often I think he actually believes it), but he's lying. He isn't any more honest than Hillary. In fact, he's almost certainly worse.