Hilarious guest stars

Which actors/actresses/celebrities made you laugh/lol/lmao when guest starring an episode of a show? Which is the show with best guest stars?

My fav is Jean Claude Van Damme's (as himself) in Las Vegas: HILARIOUS!!
But i laughted a lot with Brooke Shields in Friends as a crazy crazy fan and Summer Glau (as herself) in The Big Bang Theory

Van Damme's appearance in Las Vegas is one of the best ones in history of TV, especially for the ending of the episode...

Madonna's appearance in a SNL's gag with Lady Gaga was amazing as well.

I love Conan O'Brien on 30 Rock. Oh, and Jon Hamm too. Mmmm, Hamm....

Jack Black and Luke Wilson suddenly turning up on Community was pretty crazy too.

One of the last things did by Charlton Heston: he as himself with Alzheimer problems in a series about a nurse man (but I can't remember the title of the show, it was a lot of time ago, more than 10 years, I guess); he found him lost in the hospital and brought him to his home to show him to his friends. Mr Heston shows a great sense of humour there making jokes even about his own illness XD.

Jean-Claude Van Damme was really great there XDDDD.

Tyg wrote:

I love Conan O'Brien on 30 Rock. Oh, and Jon Hamm too. Mmmm, Hamm....

yes! conan was so funny, as always.

I realised the other day that the Late Night Wars means no more Coco on 30 Rock.