In a nutshell, SpongeBob is hard to avoid. I mean, he's everywhere: books, TV (of course), and heck, even food products. He's become a nationally-recognized icon. Though the latter seasons have declined in quality, SpongeBob, in my opinion, has never ceased to grab my interest, with the exception of a few episodes. His show is so popular, in fact, it was able to warrant its own movie in 2004, which is really saying something.
Now, all that aside, to the more technical aspects regarding the show itself, the premise is unique and the characters are full of variety, while at the same time not overdoing the amount of characters, which many shows tend to do.
SpongeBob, as you can clearly see, is the titular character: a kitchen sponge with many human characteristics, making him a much-loved character. He regards his job very important (a good quality in his character) and tries his best to make others around him happy and impressed. I think one of the reasons I've come to love SpongeBob is because he's an adult, with a job and his own house, but after all these years, he's managed to keep his child-like characteristics, always happy and cheerful and always having fun, something many adults should aspire after. Through the years, his design has changed. Back in 1999, when the show initially aired, SpongeBOB was a paler yellow and somewhat less squared, meaning his shape wasn't as straight as it is today. I think the best design was between 2002-2005. The newer design, in my opinion, is better than the 1999 version, but it comes nowhere near to the one I favor over all others. Sure, he may be somewhat a little too close to Squidward sometimes now and his personality has changed a bit over the years, but nothing, absolutely NOTHING, makes him a frustrating character. In fact, if anything, he's loveable and, though childish, a great role-model.
Patrick, on the other hand, is a complete idiot, a loser at that, but also one of the main reasons I watch this show to begin with: he's funny. If you were to ask SpongeBob fans what character he/she were to like the most, half of them would probably say Patrick. In fact, his idiocy makes him an almost clever character, with humorous lines and some pretty memorable scenes. His 1999-2000 design was a bit... offputting, which is why I prefer the newer designs, the main reason being his eyebrows, which were way too bushy in the older episodes. His personality has always been stupid: threatening to kill SpongeBob because he didn't get a Valentine's gift, suspending himself in midair with a flag in his butt and crashing right into a ceremony causing a fire, causing a plane to crash, etc. The latter episodes have turned Patrick into an idiot and nothing more, however, which makes him a tad less likeable. Sure, he's still funny, but he's too much of a giggling idiot to the point where he has almost no personality, besides being an idiot.
Squidward is the third character that comes to mind and he's unlikeable for many reasons, even more so than Plankton. Think an animated, undersea counterpart of Frasier Crane (from the sitcom, Frasier). He's bland, boring, self-centered, angry, and even depressing. Only very rarely does he show any remorse for SpongeBob or Patrick. He doesn't take his job too seriously, even falling asleep on the shift. He's oblivious to the fact that he's actually more annoying than both SpongeBob and Patrick, even though he regards himself more superior and is irritated by the duo's escapades. Basically, Squidward needs to chill. In fact, his personality has become even more offputting as the series progressed, showing little to no remorse for anyone than ever before, but he's fulfilled his purpose: being an obnoxious, uptight jerk, which what makes him funny.