I think for me it would be Alfred Russel Wallace, Darwin gets all the credit for the theory of evolution by natural selection, but Wallace presented his paper at the same time. It would be fascinating to understand how he came up with his theory which completely changed our understanding of humans’ place in the world.
I don’t know enough about enough but I’ve got huge respect for Dorothy Hodgkin (largely thanks to Georgina Ferry’s excellent Biography of her). Her area of research wasn’t mine but I love hearing about her as a great scientist, mother and peace activist.
For me, it would have to be Charles Darwin, because I’d love to discuss his book with him in the light of our knowledge of molecular biology today. It would be really interesting to hear his opinions on everything that has happened since his lifetime, such as the Human Genome Project and structure of DNA.