The very first commercial that I ever remember seeing was Wendy's "Where's the Beef?" (1984). I was 4-years old at the time, and the commercial was a big hit. It separated Wendy's from the competition and introduced the world to a catchphrase that still, to this day, is used to emphasize lack of... anything.
Here's a throwback of the three elderly ladies evaluating their burger and actress Clara Peller exclaiming, "Where's the beef?!"