If I had to pick a single favorite, I'd go with Yoda. Great original song (Lola), great parody, and its about the greatest little green dude ever.
Bohemian Polka, Albuquerque, and Ode to a Superhero (the Spiderman/Pianoman parody) are all up there too, though.
What can I say. I'm still a fanboy after all these years.