I think it depended for me on where I was in transition... Early on, the fact that I had thick wavy almost waist-length hair was the biggest help (I'd been mistaken for a woman even when I was living as a guy and had 3-day stubble). Midway through, it was a tie between the face and the fact that accenting 38DD boobs to distract from everything else seemed to go a long way. Eventually, I felt as if GRS was the key to finally being read fully as a woman, because nobody could argue if I even looked like a cis woman naked.
Now, a few years in... oddly enough, it's my voice. I look like a boring, ordinary cis woman, and I'm fine with that, but I take great comfort in being able to convince anyone as soon as I open my mouth.