OK, my favorite commercial is old. It aired in the late '70s or early '80s. It was for either Carter's or Gerber's onesies. It starred Jonathan Winters, I believe. (Hey it's been over 25 years since I last saw it.)
The commercial has Mr. Winters talking about the onesies, two babies are wearing and crawling around in, he describes how they fit and how comfortable they are, etc. The commercial ends with him saying, "How do I know all this, because," he rips open his shirt, "I'm wearing one too."
I really wish I could find that commercial.