But I don't think any of them really come close to "taking over the show," like Fonzie did on Happy Days.
Another example I thought of is a character who literally took over---Avon on "Blake's 7." It was all about Blake but Avon was just as popular and when the actor who played Blake decided to leave, Avon both literally and figuratively took over. Of course, it wasn't all due to fan interest so much as necessity.
That is a very good example. I think, from the start, Paul Darrow put more effort into his character than Gareth Thomas ever did and Avon's relationships to others (Vila, Servalan) were interesting. whereas I found Blake to be quite boring character.