This is a hard one. I can't say there's any particular person I want to meet, who is alive and kicking in todays world.
Past? I can do that. We'll pretend that he can understand my slang. Since he can receive a letter from the future and all.. Even these letters are really for me not them so :P
I would like to humbly thank you for.. everything. Your impact on my personal life has been extreme. Your vision and inventions changed the flow of history. If I could travel back in time I'd shake your hand.
Or as Spider Robinson would put it:
I, madam, I made radio! So I dared! Am I mad? Am I?
He's a bit of a fan of yours too. And more likely to get to meet you being a time traveling science fiction writer himself.
The future is even more mad then you could have envisioned. Probably only about 1/4 as interesting as it would have been if people had listened to you a bit more.