Twenty years ago, in May of 1995, Comedy Central introduced viewers to Dr. Katz, Professional Therapist, an animated series about an easygoing therapist with a quick wit and a slow pace. Viewers followed the squiggly cartoon version of real-life stand-up genius Jonathan Katz for six seasons, as the good doctor dealt with the day-to-day life of helping famous actors and comedians deal with their day-to-day lives.

Beset by his inept son Ben (H. Jon Benjamin) and icy receptionist Laura (Laura Silverman), Dr. Katz attempted to balance being both a good father and a competent therapist, which usually meant easing back and letting the characters around him rant until they felt better.

In the two decades since its premiere, Dr. Katz, Professional Therapist has developed a devoted following. Witty exchanges, likeable characters, and pretty fantastic music (largely performed by co-creators Katz and Tom Snyder) set the bar for generations of mature, low-key, animated shows.

While Katz and company serve to anchor the show through episodic story arcs and a subtle continuity, it’s the parade of delightfully demented patients that made it deeply funny both then and now. Here are 10 guests who make us glad that doctor-patient confidentiality doesn’t extend to cartoons.

1. RAY ROMANO

Dr. Katz

’s second episode featured the first of Ray Romano’s seven appearances. His surprisingly fresh riffing on well-trodden comedic topics like being a dad, marriage, and watching sports serves as reminder why Everybody Loves Raymond took off the way it did. As one of Katz’s most frequent patients—second in appearances only to the side-splitting Dom Irrera—Romano’s recurrence helped give the show greater mass appeal.

2. DAVE CHAPPELLE

Before the monumental success of Chappelle’s Show in the mid-2000s, Dave Chappelle was just another comic trying to land a few film and TV roles in between touring comedy clubs. His session with Dr. Katz, featured in the episode “Electric Bike,” is cutting-edge comedy even by today’s standards as he discusses being “Connoisseur of Racism,” terrorists’ hostage preferences, and why the Incredible Hulk is a really bad role model.

3. JULIA LOUIS-DREYFUS

One of the most hilariously awkward sessions of the entire series takes place over the phone. Julia Louis-Dreyfus does her best to deal with Katz’s wandering mind. Unfortunately, Louis-Dreyfus’s best means leaving the doctor on hold to wallow in his own awkward silence. He admits aloud to himself that a ratio of one minute of therapy to 10 minutes “in the bathroom” isn’t ideal. Maybe that’s why the future Veep star never called back.

4. JEFF GOLDBLUM

In the season six opener, the charismatic star manages to momentarily disarm the perpetually hostile Laura, as well as muse on his dreams and life as a megastar. His loose, natural conversation with Dr. Katz is insightful, uncomfortable, and organic. Even in Squigglevision, Goldblum manages to be charming.

5. LOUIS C.K.

As far as stand-up goes, Louis C.K. is pretty much an established legend at this point, who will forever be known for his ability to carefully build a clever setup, then kick it over with a smart and satisfying payoff. Since Dr. Katz was partially an animated stage for traditional stand-up material, Louis C.K. shined.

6. JOAN RIVERS

The late comedienne only appeared on the show once, but her dry take on love, pregnancy, and the practicality of pre-death cremation is the kind of comedy you can only get from a legend. She lights up Dr. Katz’s office with her trademark bite and her tiny pink dog, Spike.

7. MITCH HEDBERG

The surreal stand-up graces the grey couch twice. While fans will probably already be familiar with the material also featured on his albums, his unmistakable delivery had a magic to it that, in all likelihood, would have made him a huge star. Though Hedberg passed away in 2005, his time with Dr. Katz is one more testament to the great things that might’ve been.

8. DAVID DUCHOVNY

David Duchovny’s session begins with him trapped in the closet and ends with him delving into the nature of reality. Clad in a black shirt that simply reads “Believe,” Duchovny’s musings run so deep that they actually cause Katz to falter.

9. WINONA RYDER

Winona’s lone session is a classic episode. Mainly because there’s something so real and sadly funny about being kicked out of group therapy because you don’t have “real” problems. Plus, she walks like an alien.

10. CONAN O'BRIEN

The series’ final episode aired at the end of a Dr. Katz marathon on Comedy Central in 2002, and has one of the show’s’ best plotlines: Having already appeared in a regular couch session, Conan O’Brien returns to a wisecracking Dr. Katz. Feeling a little burned, O’Brien uses a few of the doctor’s best zingers on-air, and jealousy isn’t a good look for Dr. Katz.

All images courtesy Comedy Central DVD screen grabs.