For myself? Blue! And when my hair needs a rest, I'm good with red and black.
On others, well when it comes to the more natural colors, I'm a sucker for brunettes. But I'm also fond of red and black haired people.
As for non-natural, I'm all for going crazy. Any color can look good, as long as it fits their personality and style. Blue, pink, purple, green, red... anything really. I'm not overly fond of yellow, maybe because I associate it too much with bleach blonde, which I just don't think is a good look, but that's just my opinion. Clearly the world thinks differently, considering there are so many of them out there...