I don't think he's many people's favorite, but even people who never played the games or watched the cartoons are often able to immediately associate Pikachu with Pokemon.
At this point I suspect Pikachu ranks behind perhaps only Mario and Link in recognizability in the mainstream world among Nintendo's characters. Can't say that about any other Pokemon.
*edit* Donkey Kong may actually belong on that list, not sure... Everyone has heard of donkey kong dating back to the golden age of the arcade.