Wade Wilson: mercenary, talker, fourth-wall breaker and extremely self-aware comic book super-anti-hero. In a cinematic world where superheroes are commonplace, it makes sense for Deadpool to be the perfect tonic. A hero who isn't a hero, but is super.
Deadpool, as a character, is pretty different to anyone else. Deadpool, as a movie, is certainly unlike anything Sony, Fox, Universal or Marvel Studios themselves had ever dared to put out. It's R-Rated, with graphic violence, lots of blood, strong language and scenes of a sexual nature. With the MCU relatively sexless, Deadpool's sex-positive, kink-positive and queer-positive nature really feels like a breath of fresh air in a growingly stale superhero landscape.
And audiences loved it. They couldn't get enough of the merc with a mouth. What started as a project languishing in development hell, turned into a neutered cameo in X-Men Origins: Wolverine, and became a passion project for Ryan Reynolds, with leaked test footage the catalyst for Wade Wilson's actual cinematic debut.
Really, Ryan Reynolds deserves most of the praise. Deadpool is a character he seems born to play, and he relishes it. In fact, it's almost like Deadpool has been playing Ryan Reynolds since the early 2000s....
Deadpool's original leaked test footage can be found here