For girls, I prefer very blue eyes. I don't find the grey-blue of my sister's eyes very attractive, but richer blues are great. My first love had great blue eyes. I considered them the most beautiful eyes I'd ever seen until I met my almost-girlfriend from the summer of 2005. Hers normally looked very similar, but when I first saw her they were brighter than I thought humanly possible. When I got close I noticed there were many different hues of blue in her eyes, still intense but darker and subtler, as well as an almost emerald green central heterochromia that was much more pronounced in one eye than the other. The green was not normally visible, but would show up when something surprised her and made her pupils constrict. (Other than her eyes and a slightly squarer chin, this girl looked eerily similar to Lindsay Weir from Freaks and Geeks, in physical appearance, expressions, style, and mannerisms.)
I don't have a problem with dark brown or black eyes either, especially on Asian or Eastern European girls. I'm not a fan of hazel.
I don't think blue eyes would look very good on me. They would not be a good match for my complexion, plus I tend to think of blue eyes as somewhat feminine. For me, I think green eyes would look best. My eyes are actually brown, but I distinctly recall that in the florescent lights of the bathroom of the hospital on the day my nephew was born they appeared to be an intense dark green. Emerald eyes look awesome on me. It is very possible that I'll have green eyes eventually, although that rich a green would be unusual. My dad had brown eyes when he was young, but they where more hazel by the time I was born and undoubtedly green now, greener than my mother's eyes which have always been a sort of olive green but used to be a richer hue.