> I'm curious if anyone has noticed a significant taste difference between Japonica rice and those imported from China, Korea, or Vietnam.

Yes, but it's like somewhere between mineral water and coffee. That is, most can tell the difference when directly compared, and may even prefer one over the other, but in many cases they are interchangeable.

> Is one generally considered better tasting than the others?

IMHO, no, but you tend to prefer the type you eat most often. Going back to the coffee analogy, most people have a roast / style they prefer, but few would claim that it's better.

my mum once bought "japanese rice" from the supermarket that tasted noticeably different (and worse). had to check the label, turns out it was made in Vietnam.