Even though some of her more recent stuff like Work and We Found Love are probably played a lot more Umbrella was more career defining for Rih and will always be the song that turned her into a megastar.
I don't think she has just one.
I feel Umbrella and We Found Love are both signature songs/hits for her.
And I feel that 45S and Work also will be signatures for her that people will remember, know and sing along to for years to come. And all will be seen as 'Rihanna'.