Like the majority here, I also prefer
Japanese dramas aka 'Jdoramas', by a long mile! Like others have mentioned, jdoramas diversity of genres is one of their biggest traits and advantages over the others. Also because they're generally very straight-forward and directly to the point, the storylines tend to have unexpected twists and turns to keep the viewers interested, and are far less predictable. Because of these reasons, it just feels like you're watching a totally different drama every time. They're generally very short too, most about half the length of Korean & Taiwanese dramas, so they don't drag, but they generally don't feel rushed either because the storylines are well-composed and flows more consistently.
And not to mention, there's many japanese actors with exceptional skills.
Simply to say, a lot of jdoramas have better quality overall.
Jdoramas are by far the best for me, they're much more interesting than any of the other asian dramas, they're really the only asian dramas I watch most of the time nowadays.
I'm a huge fan of Japanese entertainment, I love their dramas, movies, music, food, animes, just their pop culture in general! I'm a self-proclaimed Japanophile!

Of course, I'm still loyal to my own culture, but I can't deny my obsession with Japanese pop culture either.
Korean dramas are ok...some were good, but they're just too repetitive and predictable, it's annoying, and what's also irritating is that the storylines tend to go around in circles before it actually gets to the point...although Korea does have alot of talented actors, but their dramas are just too cliched, it really gets boring and you gradually lose interest, that's why I don't watch k-dramas anymore.
I still occasionally watch Taiwanese idol dramas, but it's still a rarity for me...a lot of them really started to irritate me because of the dismal acting in these dramas...nowadays when I watch t-dramas, I always choose ones which have exceptional actors, or actors who at least have acceptable and above mediocre skills...a lot of the t-dramas I used to watch in the past had actors who performed terribly or were very disappointing, sometimes watching bad acting is excruciating for me...this is the main reason why I rarely watch t-dramas these days. But sadly to say, a lot of taiwanese actors are not good, mainly the young generations.
I don't watch HK dramas, and frankly, I don't like them either...I did spend about 3 years trying out over 30 different HK dramas, but every time I tried, I just could not get passed the first few eps, I found them all very boring, and each ep was so draggy, so I just gave up on HK dramas altogether...that was about almost 4 years ago, and I have not touched another HK drama ever since.
So yes, it's definitely jdoramas for me all the way!