You see many different shows and sites listing the ones for SNL, but I’ve never seen one for MadTV. I have never been a regular watcher, but I have seen a couple I really liked featuring Michael McDonalds various Jewish characters, but not sure what the all time greats are.
For me the funniest–the sketch that justifies the show’s entire existence–is when the movie is made of Miss Swan’s life, directed by Garry Marshall, starring Susan Sarandon, Dennis Hopper, and Mark Hamill in a series of escalatingly hilarious cameos.
I’m not sure if you’re looking for the classic sketches or the definitive sketches. Gump Fiction and Raging Rudolph are two classics but I don’t know how typical they are.
I was going to start a thread on this topic, but found this one and decided I’d just revive it.
Anyhoo, for classic sketches, you can’t do much better than the over-the-top Spishak ads. My personal favourites are Spishak Spishwax Car Wax and Spishak Cholestra (now with 10% less anal leakage!). The latter sketch was highly topical in a “ripped from the headlines” sense—I bet most viewers today wouldn’t have any idea what it’s spoofing, though even without that context it’s probably appealing to fans of Mr. Creosote-esque gross-out humour.
Of course, MADtv was more about recurring characters than isolated sketches. Michael McDonald’s Stuart Larkin and Alex Borstein’s Miss Swan are probably the all-time greats, though it’s hard to point to any particular sketches of theirs. (Though I’d be happy to be proven wrong. Anyone care to compile a list of must-see Stuart sketches?)
We still quote this character in our house: (sorry for the subtitles)
I need someone to handle my nuts!
I always rotfl’ed when Kenny Rogers was portrayed. The jackass sketch and his free testicular exams are memorable. 
This brings back memories! These are some of the ones that I remember, don’t know if they are classic or not.
The hyper UPS guy. Just because he’s every UPS guy ever.
Pompous ass coach. Put the dash where ever you like.
Mrs. Swan has already been mentioned but here’s Vancome Lady.
There was a running series about an angry Middle-Eastern cab driver. After 9/11, he started really emphasizing how much he loves America.
(Okay, that was off the cuff, a search shows that he is “Depressed Persian Tow Truck Man.” This is the first skit they showed after 9/11.)
Larry Flynt’s Jane Austen movie is the sketch I remember most. Whenever my friends and I are at a place where cream horns are served, we can’t resist saying “This is you!” and slurping out the cream.
GI Jew action figure from Schmattel. - YouTube
Rock-er-Byes (“Chris Rock”) - YouTube
Gangsta barbershop quartet Mad TV -Gangsta Shop Quartet - YouTube
Donnie and Marie screwing around. 
MADtv - Donny and Marie Screwing Around - YouTube
Fun with the Chads. Love the lyrics. Fun With The Chads - YouTube
I think I only ever saw two or three episodes, but I’ve always loved this Sketch with Bob Newhart as a very direct therapist. I’m convinced Dr. Phil stole his whole shtick from it.
That’s the one I came in to mention.
There’s also some excellent parody music videos. My favourites being Shakira and Britney.