which is better ? korean dramas , japanese dramas or chinese ?

Well, interesting question!

Tbh, They have their own charms tho' and you definitely can't compare them. But, If I have to choose among the three, I'd choose Korean Dramas.
They have better execution in overall aspects such as Story Line,  Casts,  Dialog, Plot, Sound Quality, Scene, and Ost although some Chinese dramas have really really great ost. They usually back every important scene with good ost. Some dramas have been really really great and captivating in every episodes till the end. They could make you linger about the story and the plot for quite a while, ofc the casts as well and that's how good and hooked Korean Drama is. 

Some Chinese/Taiwanese drama also have great quality as Korean as well such as Fated To Love You (Taiwanese Version, actually the original one before it was remake by Korean), Behind Your Smile,  A Love So Beautiful, When We Were Young and Some Day or One Day (oh this one is really really good! very high quality. Everything on them was really great).  These dramas I mentioned have really different vibe from almost every Chinese/Taiwan drama, sometimes underrated, unique and makes you think more about life. You should give it a try.

Hope it helps. :)

My vote is k-dramas. Kind of hard to chose. The other ones are different 

Japanese dramas