There are many reasons why "It's Always Sunny in Philadelphia" has lasted on air long enough to become one of the longest-running sitcoms of all time. Each season of the show, even in its weakest runs, is capable of producing moments of jaw-dropping hilarity. Where other shows hide from hot-button issues, the "Always Sunny" crew dive headfirst into the thick of it, finding humor in the darkest of subjects.

But of course, a sitcom would be nothing without its characters, and the gang at the center of "It's Always Sunny in Philadelphia" ranks among the most memorable characters of all time. The show has positioned itself as a kind of "anti-'Friends'" in which a crew of terrible people grew worse with each season. And yet, as the show has plumbed the depths of depravity these characters can get up to, it's shown a surprising amount of heart. These characters aren't just awful,...