I used to have this daydream in which I'm somehow sent back through time and get to meet Leonardo Da Vinci so he'd definetly be on the top of my list
Aside from Da Vinci, great scientists like Gallileo Galilei, Isaac Newton and - of course - Charles Darwin are people I'd like to meet.
When it comes to dead musicians, I'd very much like to spend a day with Frank Zappa. Or, although she's not dead, I'd also love to go back in time to the late 70s or very early 80s and spend a day or two with Kate Bush at East Wickham farm.