Bix Beiderbecke -- such a stupid premature death, and I think he'd really have enjoyed the musical possibilities that came after he'd died.
But actually I'd rather have a time machine to go to 1931 and haul him out of the mess he was in and get him to a hospital in say 1994 for the treatment he needed, then put him back without the pneumonia and with enough booze delivered every week for him to be able to give the wood-alcohol a miss.
Robert Holdstock, just because he was a great bloke and e.Coli is such a dire way to die too young.
There are quite a few I'd like to bring back but only if they wouldn't hurt if I did: Rosemary Sutcliff, for instance. And Diana Wynne Jones with the same proviso -- and then just to be there while those two talked together, because I think it would be an absolutely wonderful conversation.