The Spice Girls were popular when I was in high school. Elmo is five. The odds of him knowing who the Spice Girls are, or who David Beckham is must be non-existent. So it's a dumb joke, and it's awkwardly set up.
Runners-up: Red and Rover, Brewster Rockit: Space Guy!, Speed Bump
I was actually a little moved by this strip. Stephan Pastis often includes short term characters who appear for a few jokes. Most disappear within a week, some stick around like effeminate vikings and the Crocs. Andy is my favorite new character in a while. He actually has personality; this isn't a joke, it's a character sketch. I just love Andy's unrelenting hope and I love the pathos Pastis imbues Andy with.
Bonus Visual Runner-up:
It took some serious deliberation to decide whether I liked Lio or Pearls Before Swine better today. I love the drawing of Lio laughing in the third panel. And the joke is better than the jokes in most of the one off joke strips.
Runners-up: Agnes, Lio