Damn. That's a really cool sun design. It even incorporates the four cardinal directions, which I think was a major part of Native American religious beliefs, if I recall correctly.
I think, given the limitations of both 'simple' and 'unique' when there are already so many flags in existence, it's pretty darn hard to come up with a really good quality, original flag.
On another tangent, my favorite probably has to be the Jolly Roger, because, well, pirates.