I'm not picky. It just depends on the girl. Some look better with shorter hair than others. Pretty much any option is okay except for bald or like, reeeeeally short. Other than that, it varies. I am a bit biased to longer hair, however.
Personally I think the style and color are more important, also most anime girls have their own length that often times is the best for them as an idividual so I simply voted on the length of my favorite anime girl, Mizore Shirayuki.
It was a very hard decision, but medium-length hair down mid-back looks the best in my opinion. But quite honestly, hair suits the personality, it's not something that can be chosen for the general populace. In other words I love all the moe that comes in all shapes and hair lengths!!! X3