I think there have been threads about this before, if so, forgive me. Anyway, I know for a fact where Flo from Alice became bigger than Alice herself.

Flo-Alice
Fonzie-Happy Days
The Sweathogs-Welcome Back Kotter
Lenny & Squiggy-Laverne & Shirley
Urkel-Family Matters
Joy-My Name Is Earl

Larry, Darryl & Darryl - Newhart

Jack and Karen from Will and Grace

Alex P. Keaton - Family Ties. Originally, the show was supposed to be focused on the Keaton parents.

Fonzie for Happy Days. He was originally not the center or main character, sort of a side character, not a Ralph or Potsie was, as Ron Howard was most likely the main character intended at the beginning. The "coolness" effect, popularity and just the Fonz character in general turned him into the star onward (the 3rd or 4th season on), although, until his last season, Ron Howard was still A main character.

Pearl Bodine in The Beverly Hillbillies first season

Unhappily Ever After

The main characters at the start of the show were Jack and Jennie Malloy, but it was daughter Tiffany that ended up stealing scenes and becoming the most popular character on the show.

Of course, it helped that it was Nikki Cox playing the role- she was probably the curviest teenage character I've ever seen on T.V.

Good Times-About a struggling Black family in the ghetto, the JJ character got so popular he stole the show.

Good Times-About a struggling Black family in the ghetto, the JJ character got so popular he stole the show.
That's right - DYN-O-MITE JJ!! :lol:

That was why Linda Lavin got pissed off because Polly Holliday's character of Flo was bigger than Alice and she was the breakout star. If you watch Polly's final season on Alice you can see the interaction between her and Linda Lavin was not the same. You can see the tension between those two where they interacted very little on screen. At the beginning of the series you can see how close they were.

Laura Petrie on the Dick Van Dyke Show was a breakout character where she was almost funnier than Rob.

Meredith Baxter Birney was not happy how she was everything but kicked to the curb during the run of Family Ties. She was the star of the show but Michael J. Fox became the breakout star. Too bad because they could had did some nice storylines with Steven and Elyse's relationship.