I went back in January - bad timing because of a number of coaster closures, but here are the parks I went to.
- Lotte World: probably the park I'd recommend the most. This place is jaw-dropping. As Ogre already mentioned, it's Disney-esque in its level of detail and theming. Features Atlantis Adventure, which is a great coaster and really well-themed; French Revolution, possibly the best-themed Vekoma MK-1200 in the world; and don't sleep on Comet Express, one of the world's only Intamin Twist and Turns and actually a fun little coaster. This place had me dazzled all day, it's breathtaking in its scale and brilliance. It's very accessible from the Seoul Metro.
- Everland: the obvious drawcard here is T Express, but this park is quite large and very beautiful with a good selection of rides. Also, don't miss the zoo and gardens, which are changed to a different theme every season. But of course, T Express is one of the most sought-after credits in the world, so it's worth the trip for that alone. About a 30-40 minute bus ride from Seoul.
- Gyeongju World: a bit of a trek from anywhere you're likely to be visiting for any other reason, but it features a B&M Invert and a Dive Coaster. A pretty good-looking park as well, although I found it was by far the worst in Korea for its operations and upkeep. Gyeongju has a high-speed rail station, but it's another half hour or so by bus from the station to the park. Still, Phaethon and Draken would be worth the trip from Busan.
- Lotte World Adventure Busan: a brand spanking new park by Lotte, opened in 2022. It features a Blue Fire clone and a PowerSplash. The park looks fantastic and definitely has the potential to live up to its sister park in Seoul, but it's a little sparse for the time being, which is forgivable since it only just opened. I'm sure it will expand gradually. Busan is a great city, and the park is a train ride from the city, so worth a stopover if you're there.
- Seoul Land: the coasters here are awful (a couple of head-banging loopers, an abomination of a Wild Mouse that is baffling in its very existence, a Zamperla twin helix and a wacky worm), but the park itself is really very beautiful and has a large collection of rides and attractions. Entry is cheap and it's accessible from Seoul Metro, plus the location around it is absolutely picturesque, so it's worth a visit if you're looking for things to do.
I also went to Grand Children's Park for the Vekoma SFC credit, but I would not recommend that for anything more than the credit. It's poorly maintained and runs rough.