Well: There's an 日本动漫 where all the boys seem girls because the style of drawing is a little weird but 你 get used to it! This is The Wallflower 或者 Yamato Nadeshiko Shichi Henge: Kyohei, Yuuki, Ranmaru and Tagenaka This 日本动漫 is very funny!
Momiji Sohma and Yuki Sohma from Fruits Basket
Tadase Hotori from Shugo Chara
When I saw them I really thought until the end that they're girls! XD
Nice question! My answer is probably Yu Kashima from Shojo Nozaki-kun. She's a girl but looks a lot like a male, and even dresses as a prince for Drama Club. I mistook her for a crossdressing guy until I looked at the wiki. XD