A similarly themed article was posted on Onlinedegree.net -- "10 TV Characters Who Saved Their Shows." Check it out, it's a fun read.
Here are my own picks for the characters who came along and stole their shows.
Alex P. Keaton in Family Ties - The comedy was supposed to focus on the parents, a couple of middle-aged former hippies, played by Meredith Baxter and Michael Gross, as they raised their children in a world that seemed to have abandoned their free-spirited "let's change the world" ideals. The audience and eventually the writers fell in love with the oldest son, Alex, a young Conservative during those yuppie glory days, making him the center of most of the storylines. Played brilliantly be Michael J. Fox, the character overshadowed not just his parents, but his siblings as well -- the sexy but dimwitted Mallory (played by Justine Bateman) and the precocious kid sister Jennifer (played by Tina Yothers).
Fonzie in Happy Days - Arthur Fonzarelli was meant to be just a tiny character in this TV show inspired by George Lucas' American Graffiti. Instead, the leather-jacket clad anti-hero became the epitomy of "cool" and a true pop culture superstar, more recognizable than anyone else on Happy Days. Forget about Richie Cunningham and his family. Forget about Potsy, Ralph Malph, Joannie and Chachi, and all the rest. The star of the show quickly became the Fonz.
J.J. Evans in Good Times - Here's another example of a small character who delivered the best lines and generated the biggest laughs. Played by Jimmie Walker, he was truly "dyn-o-mite!" The antics of the rest of the family became just filler as viewers waited for J.J. to arrive and do his schtick.
Kramer in Seinfeld - The classic comedy was a real ensemble endeavor, but Kosmo Kramer became the most outlandish, most hilarious piece of the pie. Some might argue that any of the other castmates were better -- Jerry, Elaine, George, or even any of the supporting castmembers -- but Kramer steals practically ever scene he's in.
Steve Urkel in Family Matters - Some people don't even remember this show's name or any of the other characters on it, (or that it was a spinoff of Perfect Strangers), but nerdy Urkel, portrayed by Jaleel White was hard to forget. Annoying yet endeering, he became the face of the series.
Let me know if you think of any others.