Medium to long, straight and dark. Blond is also very alluring and it looks better short to medium. Never cared much for red but that is just me.
To me the darker haired ladies seem to have the most striking faces with the dark eyebrow and generally darker complections. Megan Fox being a prime example of this. Not to say that there aren't striking and beautiful ladies of every hair color and cut, that's just my preference.
Definitely dark brown-black. I see hot blondes and brunettes, and I see beautiful brunettes and the hot brunettes are hotter. Long and straightish, but with a bit of character, not just super straight.