I personally am a fan of Andy Richter. He does not always talk much but he can say some pretty funny and dirty things. Paul Shaeffer is good. He has quite a bit of energy compared to some sidekicks and he compliments Letterman well.

Paul Shaffer. David said if Paul weren't there, he would have ended his show a long time ago. They complement each other.

You didn't list the best one -- Craig Ferguson's robot skeleton sidekick, Geoff Peterson.

Triumph the Insult Dog

Guillermo from Jimmy Kimmel LOLLL, the jonah hill & tatum one was tooo funny ahhah

Guillermo from Jimmy Kimmel live

Andy Richter from Conan

Chuy Bravo from Chelsea lately

Steve Higgins from late night/the tonight show with Jimmy Fallon

Paul Shaffer from Late night with David Letterman