The best manhwa that nobody except me knows about is Shin Angyō Onshi (Blade of the Phantom Master in English). Although it's not on the same level, for me it's the Korean Berserk, a work with sublime drawings that, despite some slips, has greatly influenced my writing. With all its flaws, it's one of my favorite comics. I recommend it to everyone.
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Regarding Solo Leveling, I was obsessed with the novel and also enjoyed the webtoon, especially the idea of dungeons, rankings, guilds, in a modern fantasy context. I quite like the early part of the story, but it progressively gets worse, turning into a bit of a mess. I haven't watched the anime yet. I usually wait for the seasons to be complete.
I tend to prefer Japanese works in general, but I really appreciate Korean ones for often having mentally stronger protagonists who are less at the mercy of events. If I have to choose between Korea and China, I'd choose Korea any day.