South Park premieres its 21st season on Comedy Central this month (BT TV channel 307/370 HD).

Kicking the new season off with a highly controversial episode about the white supremacist rally in Charlottesville, Virginia, the animated show is still determined the push the boundaries of decency and censorship.

[Read more: South Park presents Donald Trump with a dilemma]

Cartman and the gang have frequently got in trouble with celebrities, religions, entire countries and revels in causing offence to even the most broad-minded viewers.

Ahead of the new season, we look back at the top five most jaw-dropping episodes.

South Park - Eric Cartman

5. With Apologies To Jessie Jackson

The most memorable gag from this episode is so outrageous, we can't even outline the joke broadly here without crossing a line.

It essentially revolved around the use of a word that should never be used, which South Park managed to squeeze in 43 times in a single episode.

The episode was actually praised by several groups in the black community, but it drew huge criticism from the notorious US conservative group the Parents Television Council.

4. Ginger Kids

Nobody is excluded from Matt Stone and Trey Parker’s attacks and it was ginger-haired people who were the butt of the gag in this Cartman-centered episode.

In the real world, the show was blamed for some organised attacks on red-headed students who were attacked on their way to class. Although the show’s creators would no doubt argue that it is satire and can’t be held responsible for viewers actions, it caused a bit of stink at the time.

3. Trapped in the Closet

Mocking scientology is a risky business which most comedians avoid, but South Park didn’t hold back on the topic as it made Stan play a second coming of the religion’s founder L Ron Hubbard.

Not only did the episode poke fun at the religion’s mysterious story of Xenu, it also made some legally dubious gags about Tom Cruise that the Hollywood star opted to not to “dignify” with a response.

The rumour is that Cruise managed to get the episode's repeat airing blocked, but the actor denied the claims.

South Park creators Matt Stone and Trey Parker

2. Hell on Earth

South Park’s ability to turn out output at a rapid pace is not only impressive, but also contributes to its ability to cause controversy.
This episode featured an unpleasant depiction of the late Aussie TV star Steve Irwin, which would probably have gone unnoticed if it aired six months or a year down the line.

However, airing it just days after his tragic death left many of even the show's biggest fans feeling uncomfortable.

1. Cartoon Wars

It should have been a straight forward episode, ripping gags about rival animated comedy Family Guy, but Cartoon Wars ended up causing an unprecedented storm.

The episode features an extended gag about Family Guy’s plans to show Muhammad uncensored and the timing of the episode, which came after the publication of Danish cartoons depicting the religious figure, brought a heap of issues.

Radical Muslim groups threatened Comedy Central and South Park’s creators Matt Stone and Trey Parker and the decision to place a black box over all appearances of Muhammed in the final edit upset free speech campaigners and fans.

Opening up a huge debate over freedom of speech, the episode pushed plenty of boundaries and buttons, but caused plenty of headaches and outrage along the way.

South Park returns for season 21 on Friday, September 15th at 10pm on Comedy Central, BT TV channel 307/370 HD.

[Read more: What to watch on BT TV in September]