But I don't think any of them really come close to "taking over the show," like Fonzie did on Happy Days.
Except maybe Barnabas Collins, whom Starbreaker
mentioned earlier. He didn't even appear on the show until episode #210, and was meant to be just a short-term villain, but he quickly became both the star and central focus of the show, eclipsing all the other characters.
I'm kicking myself for not thinking of him right away!