Devil May Cry is an intensely difficult series. You need to be quick on your feet and willing to go through a controller or two during some of the more difficult parts of the game if you're to survive its continuous onslaught of crazy monsters and puzzles. Mundus is one of the more intimidating devils you'll ever come across, but when the fight is over and the dust has cleared, you'll feel like you've truly accomplished something.
Fleming is an overconfident jerk that spews more cheesy lines than the Duke, but he fits in perfectly with the quirky, humorous, and sadly overlooked world of Shadows of the Damned. He'll incite as much eye-rolling as laughter, but he's also one of the more colorful takes on the devil I've ever seen.
The Darkness isn't a conventional take on the devil, but the similarities are certainly there. Even without Mike Patton's brilliant voice work, the Darkness is great because he's your own personal devil. He'll grant you amazing powers—for a price, of course—and he'll make you think you're in control until he violently tears that control away. Jackie learned this the hard way when the Darkness forced him to watch helplessly as his girlfriend was brutally murdered right in front of him.