We can all agree that Usagi is the best, right?
I've said it before and I'll say it again.
Except me back in 1995. She's pretty much the only time I've ever liked a blonde girl the best.
It kind of helps that she's a legitimately genki character who is the focus of the show. So even when she does stupid or outrageous things, the plot justifies it. If you had a character like Usagi in some random harem, she would probably be annoying. But Sailor Moon made it work because she was the focus. And because of endless reaction images. And because Junichi Sato is one of the best directors in anime.
Ami is easily my favorite, but I do admit that Usagi had wonderful character growth.