Hokkaido
I'm not a fan of winter. I'm not a particular fan of winter clothing either. That being said, I would brave all of that to experience winter in Hokkaido because from what I hear it's absolutely brilliant. The uniform in question isn't comprised of bulky winter items, but light ones instead. This creates a cute and aesthetically pleasing look while also being appropriate for experiencing winter in Hokkaido. It was an easy choice.
Osaka
Often times it's used to signal out the nerdy, quiet bookworms of an anime/manga, but I think it goes beyond that and gives a girl a refind look that no other outfit really does. I'm also fairly weak for nerdy, quiet bookworm girls -- so again this was a fairly easy choice.
Fukushima
I have to specfiy this before everything else: I think Fukushima, Tokyo, and Niigata all are good. I like them equally, but I'm picking Fukushima because of the knee high socks and ZR. I don't think the ZR gives it a distinct advantage over the others though and they are all aesthetically pleasing.
So there ya have it. There are my three. If I had to pick a single one, I'd probably choose Osaka. But, I do find the ones I picked to be equal to one another.