IIT JEE is far more tough than Suneung and Gaokao if we are on feild specific to engineeringI have tried all three(sample and previous years of Gaokao and Suneung and I have myself cleared JEE advanced years ago)Gaokao questions needs you to cram a lot... And I mean a lotHeck, even for a math question, you have to cram a lotSo Gaokao is actually a test of memory more than calculations and derived scienceSuneung is like a rapid game about just solving question effectively as possibleJEE is torture hall. First of all, a student should have ability to unwind questions into simpler form, then deduce what they really have to do and then answer it within 40 second.Even my Chinese and Korean friend were stuck and failed in it(just sample test for a try) and both of them have cleared Gaokao and SuneungBut then again, it really depends on person to person.For me, it's JEEBecause of 40second window in which you have to untangle, deduce, remember formulaes and then solve the questionBut then again, I am from the time when there was AIEEE(all india engineering entrance exam) and IIT JEE (IIT joint exam) and not today's JEE mains and advanced kind of thingsI have seen the new exam format of JEE advanced and it's tough as the old onesThe JEE mains is actually easier