(I wrote a post on Schoolhouse Rock! almost 4 years ago.)
The Wikipedia entry has a complete list.
It would be hard to pick a favorite, but one of them is Rufus Xavier Sarsparilla:
Others on my short list are: Three is a Magic Number, Interjections, Interplanet Janet, A Victim of Gravity and No More Kings.
Special bonus: Elizabeth and Wil singing No More Kings when they were 6. Wil's exuberance drowns out Elizabeth and me a fair bit ;-)