Sizz-lor_Lord of Lunch
A few years ago I would have said Lindsay Lohan... but that's before she dyed her hair, became anorexic, and degenerated into a total mess.
Now... hmm... I'll go with Kirsten Bell from Veronica Mars.
When I was a kid, I wanted to be Lydia (Winona Ryder's role in Beetlejuice.) But that was sort of when I figured out my gender didn't match my body, though I didn't know the words for that yet.
And since you asked 'gender' instead of 'sex,' I'd have to say Johnny Depp :p
(yeah, I know i kinda wear that whole thing on my sleeve, if it gets too annoying just give me a virtual slap or something.)