So therefore, not necessarily in order, we have:
--Benjamin Linus, from the TV show LOST. One of my favourite TV characters below the Doctor. He's practically the definition of "pathological liar". He's old and has the weirdest buggy eyes, but he's one of the cleverest, strangest, best acted characters around, so I love him. He also has a really cool name.
--Eugenides, from The Queen's Thief series by Megan Whalen Turner. Thief, liar, husband, king. Also short and whiney and one-handed. The Awesomest.
--The Doctor, from the TV show Doctor Who. Now this might seem somewhat controversial, but remember this: Rule #1--The Doctor Lies. Especially recently, and I love it. Even when he had a personality that wasn't quite so into lying, he still kept a darned lot to himself, which can seem a lot like lying sometimes. Plus he's constantly pretending to be other than what he is (e.g. John Smith).
--Ronald Eustace Psmith (the "P" is silent as in phthisis, psychic, ptarmigan) from the series by P. G. Wodehouse. He pretends to be a Canadian poet. He throws flowerpots at efficient secretaries. He will do anything not to do with fish (even though there is money in fish). He will assassinate your aunt. He tells lies constantly, not to obtain anything, but simply on the off chance they might produce something interesting. He is awesome.
--Alan Ryves, from The Demon's Lexicon series by Sarah Rees Brennan. He manages to be simultaneously a pathological liar AND one of the sweetest fictional guys ever (below Rory Williams, the Last Centurion, though).
--Moist von Lipwig, from the Discworld books Going Postal and Making Money by Terry Pratchett. He flies about on the wings of his lies, barely evading hanging, death, assassination, and pies in the face, while somehow remaining .... perfectly average looking. In every respect.
I think out of all these amazing conmen, he may be arguably the cleverest in the actual conman type of cleverness.
--Cassel, from Holly Black's White Cat. In this case, it's not him that I love so much as his family and the whole atmosphere of the books. And because of the use of memory in these books, he often lies unintentionally, which is interesting.
--Eponymous Clent and Mosca Mye, from Frances Hardinge's Fly By Night and Twilight Robbery. These two need to go together. With their strange, mostly selfish, affection for each other and their mutual love of words, they dash about their fictional country narrowly avoiding the consequences.
--Robin and Prudence Tremain, from Georgette Heyer's The Masqueraders. This pair of adventuring siblings infiltrates society through crossdressing, and manages to fool practically everyone. They are lots of fun, and I happen to love siblings in fiction, especially when they are actually fond of each other.
--Montmorency, from Eleanor Updale's series. Just look at the title of the first book: Montmorency: Thief, Liar, Gentleman? I love him almost more for the title of his book than for anything else, but he is an interesting character outside of his book title as well.
--Carys, from the Relic Master series by Catherine Fisher. She didn't last long enough as a conman to be officially part of the list, but she was pretty cool at first.
--Jeff Winger from the TV show Community. He's only a runner up because I'm not enamoured of his actual character. But I love his brilliant speeches which make him a conman (especially in "Paradigms of Human Memory", and the one where he tries to get Chang into the study group). And I love his interactions with the study group. He makes soapy sentiment bearable for me, because you know (and the study group knows) that he's only saying all those sickly sweet, cliched things to get something he wants. But there's still enough of something there to make it the teensy, tinsiest bit heartwarming.