The Muppet Show. When it first started, defining the area of the action with the camera's frame of view instead of the physical set was innovative for a puppet show. During the late 1970s and early 1980s the show was both aimed at adults and children, while nowadays it has become exclusively a children's show. (Do you really think that WWE's Monday Night Raw would have become such a hit in the late '90s if it hadn't picked up The Muppet Show's "ironic, postmodern backstage drama" gimmick and run to the ends of the earth with it? Same with WCW Monday Nitro. The fact that the Muppets themselves recently repaid the favor by guest-starring on Raw only clenches the argument.)
Even the jokes in the Muppet series from the 1970s and early 1980s can sometimes look very corny and tame. (Of course, since The Muppet Show was a pastiche of a variety show, sometimes they looked corny at the time, and that was the joke.)
Sesame Street. Every single children's television show today owes a tremendous debt to this program for blazing the trail. Now that everybody does it, it's hard to remember that Sesame Street INVENTED quality, research-based, curriculum-based, entertaining and educational children's TV that has a visibly ethnically diverse cast and doesn't talk down to its audience.