(I hope this thread isn't restricted to crossdressers)
I've never seriously thought about that before, as just living as female might never occur in my lifetime. My initial reaction was to start mentally compiling a list of the most beautiful celebrities, but not even considering the practical ramifications, I soon felt I would rather not entirely lose my outward identity, as our appearance has a large influence on who we are (I know your hypothetical keeps our internal self constant, but I don't think that's possible).
My next step was to do a Google Images search of, "half thai models," (this thread is revealing more of my shallow side than I would like) and the first image that appeared was of Davika Hoorne. She is Belgian/Thai, while I'm Polish/Thai, but we could pass for cousins and she is more beautiful than I could aspire to be.