I know it sounds weird, but I have to admit that I often really enjoy the commercials on the Jack Benny radio show -- especially the wonderful musical commercials by the Sportsmen (Hmmmmmmmmmmm!)
I'm kind of partial to the "Christmas Elevator" song (1948-12-19 -- "Gee that funny... Four guys running one elevator..." "Second Floor! Christmas toys for girls and boys, sweaters shirts and ties,..."). But I think "The Syncopated Clock" (1955-04-17) and "The Thing" (1950-12-03) are close seconds for me...
So, what's your favorite commercial?