I would have to go with Carmen Basilio, from what I've seen he has a god sense of humour and I love his encyclopaedic knowledge of boxing. He remembers the exact dates of his fights and also knows a great deal about other fighters' careers. I think he would have been fascinating to talk to.
Jack Johnson ,Because i don't think anyone ever actually interviewed the man..they just wrote about him ( unless i missed it)
Sam langford because his life would make a great movie ...and once again i dont think he was ever interviewed
Though id kill for the chance to meet Marciano, I'd probably like to get an interview with fighters that didn't get any or enough, like Ad Wolgast, Tommy Burns, Tiger Flowers, etc.