Aight, this is definitely the best K-drama, or in that matter any TV show I've seen! The first episode starts off mediocre but every episode after that is a ride. It gets you absolutely hooked. The pacing is perfect, has a good amount of mystery, comedy, and realism to it, and most things are fully explored in the show. The characters are extremely well written, you'll definitely feel a strong emotion towards them. It also makes you think about the harsh culture Koreans have towards education.
Another thing that makes SKY Castle stand out within K-dramas is the fact, it never gets boring, or loses its charm. Literally every other drama I see dips down after a point- This just makes you want to keep watching more.
You may think the show has a cliche ending but in my opinion it was the perfect ending, and most characters deserved the end they got. It also hints that nothing much outside SKY castle has changed, which makes the ending better.
In short- It's definitely worth a watch