1: Very smart idea to create colorful main-characters full of unique personalities and show's fulfilled creative humor.
2: the memes have saved & protected his popularity. The show also hasn't ended yet, suggesting that TV Networks and localized broadcasts from all around the world keep getting literally thousands of views solely because of SpongeBob SquarePants (typically Nickelodeon-based channels).
3: he has a very funny logic in reacting to society! SpongeBob has a very fun-loving personality, enjoys Jellyfishing and even Fry-Cooking (a job that even I'm having a hard time enjoying all that much). Lately, SpongeBob has also been coming up with very creative English Puns. And back in Seasons 1-9, SpongeBob still had a serious-side in him back then, typically when a character goes too far with their diabolical decisions, such as Mr.Krabs hunting Jellyfishes. However, this wasn't always the case, as SpongeBob did once abuse his serious-side in A Pal For Gary, but thankfully that's not a character-defining problem, even with the fact that every Season of SpongeBob (the protagonist) is actually one & the same entity (contrary to popular belief). Miscellaneous Etcetera applies to what makes SpongeBob such a relatable character (procrastinating, feeling too emotional to grow-up, and more).
4: Squidward Tentacles because he has a good mixture of a rude man who sabotages other people's plans while overreacting, vs still being a good person who often minds his own business yet he sometimes gets victimized by Bikini Bottomites instead. Despite Squidward's seriousness, even he loves being happy with fun activities too, like having talent with a Clarinet that sounds good, and avoiding to get fired because while he doesn't enjoy his job, at least getting more money than a broke arse is important (granted, Mr.Krabs very-rarely, if-ever, pays his employees, but still).
5: ever since Stephen Hillenburg was satisfied with the scripts for Seasons 1-3 followed by the 1st Movie, he left Paul Tibbit to take care of the show by doing the work for him, and while I cannot fathom why so many people kept constantly complaining about this show's slightly altered formula back in the 2010s, I do appreciate that Paul Tibbit did at least try what script ideas came naturally before Stephen Hillenburg temporarily returned to the show completely back in 2015-2018. Ernest Borgnine was getting so old that he eventually passed-away of kidney-failure back in 2012, thus deeming both Mermaid Man & Barnacle Boy as cameo-characters or retired-characters rather than side-characters. Bubble Bass used 2B forgotten in the middle of the series, but was eventually restored in Season 8 after his long absence streak in Seasons 2-7. Plankton eventually got a new pet, who is a Dog-like Slime that can shapeshift forms and eventually the other additional recurring-characters for this series/show would happen 2B none other than those introduced in Kamp Koral, which was developed while this controversial backstory was revealed regarding Stephen Hillenburg's consent in making SpongeBob SquarePants spin-offs (probably the worst situation to ever happen right after Stephen Hillenburg's tragic death).
6: Probably Seasons 1-3 to get the idea of what makes SpongeBob SquarePants so iconic. Then continue with Seasons 4 & 5. Then continue with Seasons 8 & 9. I would recommend them to skip Seasons 6 & 7 until they've watched all of the better seasons. Then eventually, I would recommend them to follow Seasons 6 & 7 by watching Sponge Out of Water, and then eventually finishing that marathon-ish with the 1st SpongeBob SquarePants Movie.
