In my opinion, Easiest = Japanese. Middle =Chinese. Hardest= Korean
Japanese is a perfect blend of characters and the alphabet. Pronunciation and listening are not challenging. The hardest parts are grammar and vocabulary. There is also a lot of material to learn.
Chinese Grammar doesn’t seem too bad. The tones are scary and take practice, but they’re not too bad either. Speech is slower, and grammar seems pretty straightforward. What makes it challenging for me is the number of characters you need to learn (double if you’re learning both traditional and simplified) and the variations in dialects.
Korean - By far the hardest language I’ve learned. Speech changes, verb conjugations, sound linking, and pure speed of spoken speech. The only easy thing about Korean is the alphabet.
A little Background:
The US government classifies languages in categories 1-4, with Cat 4 being the hardest for English speakers. Those languages are: Arabic, Chinese, Japanese, and Korean. I’ve had the pleasure of dabbling in all 4. Arabic for 3 months before going to Egypt. I would say an upper A1. Japanese for 3 years in high school and two trips to Japan. A2 because, let’s be real, High school doesn’t really teach languages well. Korean at an upper B1, maybe B2, after learning at a rigorous language school for 7-10 hours every day for 18 months. I just started learning Chinese a few months ago. My tutor said I am all over the board, probably an A2.