12. Timmy Burch
Timmy's introduction gave Trey Parker and Matt Stone a chance to open up options of discussing disability in the guise of comedy. To do so has meant that they've featured surprisingly compelling moments and opened up many narrative avenues. Thankfully they don't mistreat Timmy, as the other kids all like him, and he seems to be an incredibly happy chap, making him irresistibly fun to watch.
With that being said, he ranks lowest thanks to the fact that the setup of his character inherently means that the joke gets old. He can only say the word Timmy, and whilst it is funny for a surprisingly long time, the joke grows a little stale. From there, he becomes more of a character, with motives and usefulness, but the limited speech choices become troublesome.
He will always feel like a lesser character since they decided to limit the amount of vocal interaction that he could have with the rest of the cast.