I have never seen one I liked... because all of them are about 'male plus female'. Various hybrids of the signs for Venus and Mars, and pastel pink and blue. The pastel, especially, bugs me - it becomes something about reinforcing the gender roles of 1950s America. "It's okay to pick your own role", these colors say, "as long as it's one of the two approved ones".
To make matters worse, 'pastel' always feels feminine to me; if I was a FtM, I'd want some much butcher colors!
With that in mind, and a joking suggestion of stealing the Trans Am logo, I doodled in Illustrator.
Not quite there in colors or execution, but I think it's a start on something very different. It's not about the male/female chasm; it's about the rebirth. Proud, bright colors for the fire; an empty white slate for the phoenix, breaking out of the greyness of their previous life. And something firey like a phoenix is a reminder to get out and do something now that you're no longer spending every minute maintaining the pretense of the gender you were born in.
Also it works hung vertically or horizontally, which is a plus.
Comments? Especially from those of you who're on the female-to-male path - does this speak to you more or less than existing designs?
edit: I made some variants.
Top to bottom: cool grey ground, rather than warm grey; two versions of 'well, we're all pretty queer, so why not swipe the six-color rainbow of the gay pride flag?'.