There are so many flavors in citrus, you will get many answers.
I think there are strong regional variations, so what tastes best in one area may not taste so good from another, the exception being limes which I love from anywhere and are probably near the top of the list.
From Cali I most enjoy the 88-2 which has a distinct musk similar to strawberry guava, but also a chalky body. I haven't tasted Allspice tangelo though, which may trump it.
From Japan, I most enjoy the Dekopon, which has many subtle layers of flavor, but really well balanced sweetness/acidity.
That said, there are so many other great citrus out there, which may have very small areas which they really reach their potential in, so it's worth trying whatever you can get a hold of.
Being in Korea, you probably don't have access to limes as easily as elsewhere, I'd say when I was in Japan, I sought those most.
I dislike the naval oranges and mandarins that make up the vast majority of the citrus sold in the US. Commercial varieties of mandarins may be easy to peel but have had all the flavor bred out of them. Naval oranges are harder to peel but only slightly more flavorful. I seem to be in the minority among American consumers when it comes to my opinion on naval oranges and mandarins. When they are available, I purchase Valencia oranges, which still have enough flavor to be worth eating.
Definitely.
I think a lot has to do with trying to get the tree to hold enough fruit to maturity/ greatest yield/acre.
In SoCal, my favorites are Navels from trees in yards which are not watered at all, the trees drop a lot of fruit, but they are intense.
Valencias have great spiciness, they really are an undervalued fruit.