Yes, The Simpsons is a fictional show. But they have a few shows within the show, or other fictional characters, and some of them are pretty damn funny. So here's a Top 5 list for that.
5. Worker and Parasite
Nothing against the incredibly violent duo Itchy and Scratchy, another show within the show which doesn't appear on this list. If anything, it's a testament to the fact to the strength of Itchy and Scratchy that there's so little quality cartoon material in the Simpsons world that Krusty has to resort to material like this when he loses everyone's favorite cat and mouse duo to The Gabbo Show. It's hilariously random, and the only clues to the meaning of the video are the line of peasants and the increasingly aggravated growls of one of the duo. It's not even clear which one is Worker and which one is Parasite.
McBain is an action star in the style of Arnold Schwarzenegger, quite an obvious analogue given his brawn and thick accent. The early examples had him in movies chock full of cliches such as partners dying just before retirement and a renegade cop who uses unorthodox methods that always aggravate his chief, like so:
This is a great example of the classic Simpsons episodes supplying high-quality humor in under half a minute. McBain's complaint about not being able to avenge his partner's death with a "pea-shooter" is funny enough, the chief's rant about his hand cannon is also great, and it's all topped by the ridiculously over-the-top way of getting around the book. Strangely enough, there actually was a McBain movie starring Christopher Walken and Luiz Guzman, among others:
If Wikipedia is correct, The Simpsons had actually used the McBain name before this movie came out, but somehow the movie studio (20th Century Fox no less) was able to prevent them from further use of the name. I'm not sure how accurate that is, considering I had an old Simpsons comic that clearly involved a McBain storyline and I doubt 20th Century Fox would suppress a name on a successful series in favor of a mediocre action flick it skillfully parodied before it even existed. Whatever the case, McBain is generally seen more in the guise of his actor, the awesomely named Rainier Wolfcastle, whose characteristics include offering to hug his daughter with his muscles and weeping furtively in his Humvee after a couple of random party guests tell him how much the last McBain movie sucked.
The McGarnagle character has only been used twice, and wasn't even seen in one of those, but I'm pretty sure every Simpsons fan will be able to recognize the name and perhaps even quote the scenes verbatim. McGarnagle kind of took over for McBain in the renegade cop category, perhaps after the writers realized that Austrian action stars are better suited for shoot-em-up films. So McGarnagle follows the same model in the early McBain clips (one terrific exchange: "You busted up that crack house pretty bad, McGarnagle. Did you really have to break so much furniture?" "You tell me, Chief. You had a pretty good view from behind your desk."). I also saw these long before I saw the Dirty Harry movies, so although I enjoyed them I couldn't help but think of this scene when I saw Clint Eastwood in action:
Apparently McGarnagle is the spelling that has been settled on, but the pronunciation of the R as "ah" has led to plenty of different spellings such as the McGonicle of this Spanish version. Even they must know there's an R in there, because the way the voice actor pronounces McGarnagle is just superb.
2. Mr. Sparkle
Mr. Sparkle has only one major appearance on the show, in an episode whose subplot involves Homer and Bart and Lisa discovering a box in the dump with a foreign language and a strange cartoonish (more so) version of Homer on it. They eventually discover that it's a box of Japanese dishwasher detergent whose logo, by coincidence, looks like Homer via a combination of a light bulb and fish.
I remember laughing for ages when I first saw the Mr. Sparkle commercial. A parody of the more over-the-top Japanese TV offerings, it's ridiculously high-energy and nonsensical. There's also the oddity of a detergent company being sponsored by electricity and fishing interests, as well as the nice related joke of Homer asking for (and immediately getting) a specific Japanese phone book from the library and then claiming that he wants to make a "local" call.
1. Amendment to Be
Another Itchy and Scratchy replacement. After the show is pulled due to the discovery that the characters were plagiarized, Krusty is left scrambling for a new animated offering...and dredges up something so bad that he begs the kids to stay on for the toy commercials he'll have. It's an obvious parody of Schoolhouse Rock and the overly conservative slant thrown in is quite amusing as well.
I guess it's less funny now that Ted Kennedy is dead. No, it's actually still pretty funny.