Well,
Ayumi Hamasaki is, in my eyes, Queen of Reinvention for Japan, hah. She literally changes with every album, and there have been many times that it was drastic, yet it's always worked.
I don't think Kumi has reinvented herself fully ever. She's been pretty much exactly the same since the very beginning, hah. Goofy girl in a sexy costume, making pop and R&B/hip-hop music with techno/electro inspiration. It's not a bad thing that she's always been like this, but she doesn't belong on this poll, I don't think.
And Amuro.... I've stated my opinion about her before, so I won't go much into it. But even though she has gone through a few really big career changes, it seems more of a sell out than a reinvention for any reason.
As for Western artists, both
Britney Spears and
Christina Aguilera had similar reinventions, but they were both huge. And it somehow worked very fluidly, hah.
Madonna has reinvented so many times, of course, just like her follower Ayumi.
Michael Jackson..... He's somebody who either ended up reinventing himself for the worse, or he just tried really hard to reinvent... Either way, I'll vote for him still, because it's the thought that counts. XD
I honestly haven't ever noticed Beyonce reinvent herself at all. Same with Kylie Minogue. (But of course, I have never payed attention to the latter--but still!
)