The devil has never been used well as a fictional character in my experience. Granted i haven't seen every movie nor read every book, etc. so maybe there's a decent story out there. But the few I've seen, his plans never made any goddamned sense, if the devil had a plan.
The prototypical devil story is the Faustian contract. The devil promises riches, power, what-have-you in exchange for your soul. In order to make the story interesting the devil always twists the fulfilled desire in some nasty way. This was most recently used in the film Bedazzled with Brendan Frasier and that hot chick. This is a stupid story because it doesn't make any sense for the devil to twist the desire. He'd get more souls if it was known he was good for his word. Fuck people over and they'll tell others to not shop there. At the end of the day, it's a morality parable instructing people to not give in to temptation.
The worst devil story, or most disappointing, I had ever read was "Man In the Black Suit" by Stephen King. It started off interesting enough with a young boy going fishing and the devil himself comes out of the woods and starts talking to the kid. There was some tension with the kid recognizing the devil but not wanting to let on because he figured the devil would attack if it knew he knew. But then the devil turned into a multi-fanged monster and tried to eat the boy.
How. Fucking. Boring.
So the devil is 8usually turned into some sort of emphatic deceiver or a run-of-the-mill monster. Neither of these things give any real sense of the pure evil the devil represents.
But the worst devil story I've ever seen was John Carpenter's Prince of Darkness. In that, the devil is a bong water monster. What silliness.