South Park kicked off in 1997, putting Comedy Central's star on the map in a major way and introducing Trey Parker and Matt Stone as two atypical Hollywood voices. The first season alone tackled genetic engineering, homosexuality, obesity, mortality, world hunger, nose jobs, sexual freedom, and a Christmas-loving chunk of talking feces.
The series has changed a lot, including the theatrical film South Park: Bigger, Longer & Uncut, which was released during the third season's initial run. And with Paramount+ releasing a series of movies, four of which were released throughout 2020 and 2021 (comprising the show's 24th season), it looks like Redditors will have plenty more contenders for the top spot down the line.
Season 4
Going into the year 2000 was a pseudo big deal for all of America, South Park included. The first four episodes of the fourth season—"The Tooth Fairy's Tats 2000," "Cartman's Silly Hate Crime 2000," "Timmy 2000," and "Quintuples 2000"—incorporated the year in the titles. The show was embracing the new century with a willingness to push the envelope even further, and it hit more often than it missed.
Furthermore, Season 4 was important for introducing Timmy Burch, a character that would run dry fairly quickly, but Parker and Stone would find some redemption for his two-dimensional portrayal later on. Polarising rattled off a few examples with "NAMBLA, Cartman's Silly Hate Crime, Fingerbang."
Season 5
Trolling_them_softly replied to an OP looking for the best season with a simple "Season 5." They then elaborate, starting off with an example that doesn't exactly reference one of South Park's surprisingly clean quotes
In their words, it "Kicks off with an episode full of the word "sh**". More Big Gay Al, and the introduction of Jimmy. Cartman reaches new levels of depravity. Red rocket! Don't forget to bring up Towelie. We lose Kenny, for real. And Butters gets his own episode. Also: David Blaine cult, Terrance and Phillip, hemorrhoids and theme parks, Osama Bin Laden, Token is a lion, Ben Affleck."
Season 6
Season 6 is important for not featuring Kenny. It's also important for what it did with the parka-clothed rapscallion's absence. Namely, the sixth season converted Butters Stotch to a main character. There were also solid individual episodes, such as "Free Hat," and the excellent exploration of Stan and Cartman's dynamic, "My Future Self n' Me."
A now-deleted user agrees, considering they wrote "For me, it was definitely season 6. 'Asspen,' 'The Death Camp of Tolerance,' 'The Biggest Douche in the Universe,' and 'The Return of the Fellowship of the Ring to the Two Towers' are some of my favorite episodes. That whole season is downright hilarious though."
Season 8
South Park's eighth season is phenomenal for several reasons. First, it was the beginning of a shift for the show from smart but crass adult animated comedy to top-tier political and social satire, which wasn't something the show had neither shied away from nor embraced in full. Second, production on the eighth season ran concurrently with the production of Team America: World Police, which was itself an intensive and grueling process. Finally, most of the 14 episodes ended up being fantastic (as did Team America).
Sulky22 said as much on a thread devoted to finding the best season for someone who doesn't have time for them all: "Season 8 is filled with classic episodes - Good Times with Weapons, Passion of the Jew, Awesom-O and Woodland Critter Christmas to name only a few. There's not really a dud episode in that run."
Season 10
"A Million Little Fibers" is notoriously one of South Park's least regarded episodes, "Stanley's Cup" is a little dark, and the "Go God Go" duology doesn't quite work, but Season 10 is also rife with classics: "Manbearpig," "Make Love, Not Warcraft," "Miss Teacher Bangs a Boy," and the two "Cartoon Wars" episodes.
One Redditor, flabflab, brought up the season's highlight, "Make Love, Not Warcraft:" "...easily the best season just because I have such a bo*** for this episode." On a separate thread, jdoss42 brought up the season with less rosy phrasing: "Season 10 is my personal favorite, World of Warcraft, 'Manbearpig,' a raging clue, Chef coming back, cartoon wars, underrated 'Tsst!' episode, Ike getting with his teacher. I love that season."
Season 11
A now-deleted user championed the 11th season, writing "'With Apologies To Jessie Jackson,' 'Cartman Sucks,' 'Lice Capades' (which I liked because it followed Clyde), Le Petit Tourette (one of my favorites ever), 'More Crap', and ending with the awesome 'The List.' Plus it had 'Imaginationland,' which I didn't think was amazing, but it was still pretty good."
"Lice Capades" is a choice many fans would disagree with, which is probably why the Redditor qualifies it, but the episode is something of a cult episode that gets better with subsequent viewings. But that episode is nothing compared to the pure hilarity of "Le Petit Tourette," which explores one of South Park's better angles—Cartman being horrible and getting his comeuppance—to a phenomenal degree.
Season 13
There's an argument to be made that Season 13 was the last season of South Park to feel like the excellent run of Season 5 to Season 9. The show hasn't outworn its welcome or completely lost its relevancy, but there's a spontaneity in Season 13 that's mostly lacking in Season 14. The season opener, a Jonas Brothers parody titled "The Ring," is just one example. While the season misses on occasion ("Pee" and "Eat, Pray, Qu***"), "Fishticks," "Butters' Bottom Bi***," and "The F Word" are classics.
Abszaa9 wrote: "13 was my favorite. I mean with 'Fishsticks,' 'Fatbeard,' 'Whale Wh****', 'Dead Celebrities,' and 'Dances with Smurfs.' None of which were my favorites of all time, but on a whole, I think it's a pretty solid season." The Whale Wars parody episode is one of South Park's best.
Season 14
Some would call South Park Season 14 the beginning of an inevitable decline, but kryptkeeper17 liked it, writing "I'd say Season 14. 'Sexual Healing,' 'The Tale of Scrotie McBoogerballs,' '200/201,' the coon trilogy, 'Créme Fraiche,' and there's more."
"Sexual Healing" is a fine but bland season opener, but "The Tale of Scrotie McBoogerballs" is a classic. However, "200" and "201" were ultimately censored, and it was an ugly precedent to have been set. In the end, those two episodes (especially "201") don't have the impact that was intended, and the season has some very weak episodes like "Crippled Summer," "You Have 0 Friends," and even the whole "Coon" trilogy. However, while Cartman riding on the back of Cthulu felt like a shark jump, it was immediately followed by the season's closer: the excellent Randy-centric episode "Créme Fraiche."
Season 16
The same now-deleted user who enjoyed Season 11 also cited Season 16 as one of their favorites, then elaborated on which episodes worked for them: "'Reverse Cowgirl' was great, so was 'Cartman Finds Love,' and it had 'Sarcastaball', which I think may be the most instant 'classic' episode in a long time. It's been a few seasons since I've seen an episode and thought right away 'instant classic."'
The Redditor mentions some good highlights, but there's also "A Nightmare on FaceTime," which features Randy Marsh in one of his stranger jobs: the owner of a Blockbuster Video.
Season 19
One Redditor started a thread to find out which is the best overall season of South Park. Baits04 replied, "As a whole season I'd say Season 19." TheChonkyDonky agreed, but with a caveat: "I remember watching Season 19 and thinking 'Da**, South Park still got it.' Then Season 20 happened...and the whole new serialization format just crumbled."
There's an argument to be made that South Park never should have been serialized, to begin with. Each episode had started fresh, where everything in the world was up for targeting. South Park is indiscriminate in its parodying, and serializing it limits the number of topics Parker and Stone can satirize.