While the premise has been run into the ground by now, the first ten or so years of these were pure gold. The Simpsons, although cartoons, still have to adhere to some basic rules of reality. Mortality is a real enough thing in the Simpsons Universe (see Bleeding Gums Murphy or Maude Flanders) so wacky stories where a killer clown doll attacks Homer or Mr. Burns turns the family into vampires don't really work in the context of the weekly series. By introducing the Treehouse of Horror concept, initially it was Bart and Lisa telling scary stories to pass the time on Halloween, the show opened itself up for more outright parody and horrible violations of their beloved characters.
Some standouts include:
Clown Without Pity- a Child's Play takeoff with a murderous Krusty Doll.
The Shinning- A Shining parody that hits all the high points of the movie while being very funny. "No TV and no beer make Homer something something..."
The Devil and Homer Simpson- This introduced Daddy's Soul donut and saw what would happen if Homer sold his soul to Devil Flanders for a donut.
Homer Cubed brought 3-D animation to the Simpsons as Homer falls into another dimension trying to avoid Patty and Selma.
Citizen Kang is still relevant to this day as it focused on the 1996 presidential election. Clinton and Dole are replaced by alien invaders but what can you do? It's a two-party system.
There are a ton of others I love: Time and Punishment, Bart Simpson's Dracula and The Genesis Tub (I've created Lutherans!) just to name a few. If you were ever a fan of the Simpsons and you've never caught these classic episodes, you owe it to yourself to track these down and check them out.