Without a doubt,
Aino Minako/Sailor Venus from Sailor Moon. To the point that her particular hairstyle (was it called the 'princess hairstyle'? long hair down but side bangs tied back) becomes THE hairstyle as my preference for girls, even IRL.
Main reason why I got back to anime was because after my hiatus from anime, they started airing Gundam Wing in my area. And lo and behold, you have
Relena Peacecraft who sports similar hairstyle as Minako lol.
Oh yeah, now that its been mentioned, I think even the Oversized Ribbon, as rare as it is IRL, is still one of the things I find attractive in anime. Again, also because of Mina-chan <3. While you're at it, maybe it's also the reason why I prefer serafuku over blazer uniforms on girls. How's that for a first crush?