How to say all-time legendary in Korean
Korean: 역대급[ yeok-dae-geup ]

Meaning
역대급 means something is on an "all-time" level, like the biggest, best, worst, or most amazing ever. People use it when they feel very strongly about something and want to say it is beyond normal. It is casual slang, so you will hear it in daily conversation, online, and in reviews.
Example sentences
오늘 날씨가 역대급으로 좋았어.
The weather today is really great.
이 음식점 떡볶이는 역대급이야.
This restaurant's ddakji is amazing.
오늘 집이 역대급으로 더워.
The house is really hot today.
이번 시험은 역대급으로 어려웠어.
This test was really hard.

